webgl: Zero-initialize the SyncToken before calling GenSyncTokenCHROMIUM

The GenSyncTokenCHROMIUM function may fail if it generates an error, or
the context already has an error on it. At that point the sync token
will not be updated, so we should have it initialized to 0 for this
case.

R=kbr@chromium.org
BUG=629008

Review-Url: https://codereview.chromium.org/2162673002
Cr-Commit-Position: refs/heads/master@{#406162}

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

@@ -30,6 +30,7 @@
 
 #include "platform/graphics/gpu/DrawingBuffer.h"
 
+#include "gpu/GLES2/gl2extchromium.h"
 #include "gpu/command_buffer/client/gles2_interface.h"
 #include "gpu/command_buffer/common/capabilities.h"
 #include "platform/RuntimeEnabledFeatures.h"
@@ -569,7 +570,7 @@ bool DrawingBuffer::copyToPlatformTexture(gpu::gles2::GLES2Interface* gl, GLuint
     const GLuint64 fenceSync = gl->InsertFenceSyncCHROMIUM();
 
     gl->Flush();
-    GLbyte syncToken[24];
+    GLbyte syncToken[GL_SYNC_TOKEN_SIZE_CHROMIUM] = { 0 };
     gl->GenSyncTokenCHROMIUM(fenceSync, syncToken);
     m_gl->WaitSyncTokenCHROMIUM(syncToken);