compositor-worker: Share a thread and an isolate for compositor workers.

Source/modules/modules.gypi

 {
   'includes': [
     '../core/core_generated.gypi',
     'modules_generated.gypi',
   ],
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
     # from time to time as the code generator changes in backward incompatible
@@ skipping 613 matching lines @@
       'cachestorage/CacheStorageError.cpp',
       'cachestorage/CacheStorageError.h',
       'cachestorage/GlobalCacheStorage.cpp',
       'cachestorage/GlobalCacheStorage.h',
       'cachestorage/InspectorCacheStorageAgent.cpp',
       'cachestorage/InspectorCacheStorageAgent.h',
       'compositorworker/CompositorWorker.cpp',
       'compositorworker/CompositorWorker.h',
       'compositorworker/CompositorWorkerGlobalScope.cpp',
       'compositorworker/CompositorWorkerGlobalScope.h',
+      'compositorworker/CompositorWorkerManager.cpp',
+      'compositorworker/CompositorWorkerManager.h',
       'compositorworker/CompositorWorkerMessagingProxy.cpp',
       'compositorworker/CompositorWorkerMessagingProxy.h',
       'compositorworker/CompositorWorkerThread.cpp',
       'compositorworker/CompositorWorkerThread.h',
       'credentialmanager/Credential.cpp',
       'credentialmanager/Credential.h',
       'credentialmanager/CredentialManagerClient.cpp',
       'credentialmanager/CredentialManagerClient.h',
       'credentialmanager/CredentialsContainer.cpp',
       'credentialmanager/CredentialsContainer.h',
@@ skipping 804 matching lines @@
       'webaudio/AudioBasicProcessorHandlerTest.cpp',
       'webaudio/ConvolverNodeTest.cpp',
       'webaudio/DynamicsCompressorNodeTest.cpp',
       'webaudio/ScriptProcessorNodeTest.cpp',
       'webaudio/StereoPannerNodeTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }