Add a call to DescheduleUntilFinishedCHROMIUM to WebGL.

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

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
index b2e3b095eb3d3e5e3f3c109f0580164ccc43a924..32416b2befbeabbe7bb114d8140b223cad285ab8 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -292,6 +292,8 @@ bool DrawingBuffer::prepareMailbox(WebExternalTextureMailbox* outMailbox, WebExt
 
     m_gl->ProduceTextureDirectCHROMIUM(frontColorBufferMailbox->textureInfo.textureId, frontColorBufferMailbox->textureInfo.parameters.target, frontColorBufferMailbox->mailbox.name);
     const GLuint64 fenceSync = m_gl->InsertFenceSyncCHROMIUM();
+    if (RuntimeEnabledFeatures::webGLImageChromiumEnabled())
+        m_gl->DescheduleUntilFinishedCHROMIUM();
     m_gl->Flush();
     m_gl->GenSyncTokenCHROMIUM(fenceSync, frontColorBufferMailbox->mailbox.syncToken);
     frontColorBufferMailbox->mailbox.validSyncToken = true;