replace flaky DCHECK in DrawingBuffer with early-out

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 03530d5f11a032dcd567d80ab8472752b7fb404e..547c1341da87bcb9d927d52d58904ba1ebbc682f 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -713,7 +713,11 @@ bool DrawingBuffer::copyToPlatformTexture(gpu::gles2::GLES2Interface* gl,
     m_gl->GenSyncTokenCHROMIUM(fenceSync, produceSyncToken.GetData());
   }
 
-  DCHECK(produceSyncToken.HasData());
+  if (!produceSyncToken.HasData()) {
+    // This should only happen if the context has been lost.
+    return false;
+  }
+
   gl->WaitSyncTokenCHROMIUM(produceSyncToken.GetConstData());
   GLuint sourceTexture =
       gl->CreateAndConsumeTextureCHROMIUM(target, mailbox.name);