Update from https://crrev.com/316786

mojo/cc/context_provider_mojo.h

Index: mojo/cc/context_provider_mojo.h
diff --git a/mojo/cc/context_provider_mojo.h b/mojo/cc/context_provider_mojo.h
index 014b9734e0fe9a0e33899c519773fb01d348b7bf..9b55a85219c68b0500d0a1cf1115dc6c9fe2b4fe 100644
--- a/mojo/cc/context_provider_mojo.h
+++ b/mojo/cc/context_provider_mojo.h
@@ -6,6 +6,7 @@
 #define MOJO_CC_CONTEXT_PROVIDER_MOJO_H_
 
 #include "base/macros.h"
+#include "base/synchronization/lock.h"
 #include "cc/output/context_provider.h"
 #include "mojo/public/c/gles2/gles2.h"
 #include "mojo/public/cpp/system/core.h"
@@ -31,6 +32,8 @@ class ContextProviderMojo : public cc::ContextProvider {
   void SetMemoryPolicyChangedCallback(
       const MemoryPolicyChangedCallback& memory_policy_changed_callback)
       override {}
+  void SetupLock() override;
+  base::Lock* GetLock() override;
 
  protected:
   friend class base::RefCountedThreadSafe<ContextProviderMojo>;
@@ -47,6 +50,8 @@ class ContextProviderMojo : public cc::ContextProvider {
   MojoGLES2Context context_;
   bool context_lost_;
 
+  base::Lock context_lock_;
+
   DISALLOW_COPY_AND_ASSIGN(ContextProviderMojo);
 };