--- a/dom/media/gmp/DecryptJob.cpp
+++ b/dom/media/gmp/DecryptJob.cpp
@@ -1,17 +1,32 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "DecryptJob.h"
+#include "mozilla/Atomics.h"
 
 namespace mozilla {
 
+static Atomic<uint32_t> sDecryptJobInstanceCount(0u);
+
+DecryptJob::DecryptJob(MediaRawData* aSample)
+  : mId(++sDecryptJobInstanceCount )
+  , mSample(aSample)
+{
+}
+
+RefPtr<DecryptPromise>
+DecryptJob::Ensure()
+{
+  return mPromise.Ensure(__func__);
+}
+
 void
 DecryptJob::PostResult(DecryptStatus aResult)
 {
   nsTArray<uint8_t> empty;
   PostResult(aResult, empty);
 }
 
 void

--- a/dom/media/gmp/DecryptJob.h
+++ b/dom/media/gmp/DecryptJob.h
@@ -10,31 +10,25 @@
 #include "mozilla/CDMProxy.h"
 
 namespace mozilla {
 
 class DecryptJob {
 public:
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(DecryptJob)
 
-  explicit DecryptJob(MediaRawData* aSample)
-    : mId(0)
-    , mSample(aSample)
-  {
-  }
+  explicit DecryptJob(MediaRawData* aSample);
 
   void PostResult(DecryptStatus aResult,
                   const nsTArray<uint8_t>& aDecryptedData);
   void PostResult(DecryptStatus aResult);
 
-  RefPtr<DecryptPromise> Ensure() {
-    return mPromise.Ensure(__func__);
-  }
+  RefPtr<DecryptPromise> Ensure();
 
-  uint32_t mId;
+  const uint32_t mId;
   RefPtr<MediaRawData> mSample;
 private:
   ~DecryptJob() {}
   MozPromiseHolder<DecryptPromise> mPromise;
 };
 
 } // namespace mozilla
 

--- a/dom/media/gmp/GMPCDMProxy.cpp
+++ b/dom/media/gmp/GMPCDMProxy.cpp
@@ -32,17 +32,16 @@ GMPCDMProxy::GMPCDMProxy(dom::MediaKeys*
                          bool aDistinctiveIdentifierRequired,
                          bool aPersistentStateRequired)
   : CDMProxy(aKeys,
              aKeySystem,
              aDistinctiveIdentifierRequired,
              aPersistentStateRequired)
   , mCrashHelper(aCrashHelper)
   , mCDM(nullptr)
-  , mDecryptionJobCount(0)
   , mShutdownCalled(false)
   , mDecryptorId(0)
   , mCreatePromiseId(0)
 {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_COUNT_CTOR(GMPCDMProxy);
 }
 
@@ -705,17 +704,16 @@ GMPCDMProxy::gmp_Decrypt(RefPtr<DecryptJ
 {
   MOZ_ASSERT(IsOnOwnerThread());
 
   if (!mCDM) {
     aJob->PostResult(AbortedErr);
     return;
   }
 
-  aJob->mId = ++mDecryptionJobCount;
   nsTArray<uint8_t> data;
   data.AppendElements(aJob->mSample->Data(), aJob->mSample->Size());
   mCDM->Decrypt(aJob->mId, aJob->mSample->mCrypto, data, aJob->mSample->mDuration);
   mDecryptionJobs.AppendElement(aJob.forget());
 }
 
 void
 GMPCDMProxy::gmp_Decrypted(uint32_t aId,

--- a/dom/media/gmp/GMPCDMProxy.h
+++ b/dom/media/gmp/GMPCDMProxy.h
@@ -213,23 +213,16 @@ private:
   GMPDecryptorProxy* mCDM;
 
   UniquePtr<GMPCDMCallbackProxy> mCallback;
 
   // Decryption jobs sent to CDM, awaiting result.
   // GMP thread only.
   nsTArray<RefPtr<DecryptJob>> mDecryptionJobs;
 
-  // Number of buffers we've decrypted. Used to uniquely identify
-  // decryption jobs sent to CDM. Note we can't just use the length of
-  // mDecryptionJobs as that shrinks as jobs are completed and removed
-  // from it.
-  // GMP thread only.
-  uint32_t mDecryptionJobCount;
-
   // True if GMPCDMProxy::gmp_Shutdown was called.
   // GMP thread only.
   bool mShutdownCalled;
 
   uint32_t mDecryptorId;
 
   PromiseId mCreatePromiseId;
 };