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GpuCommandBufferMsg_SetLatencyInfo is used to send current frame's latencyinfo
to gpu. We can piggyback the latencyinfo in GpuCommandBufferMsg_AsyncFlush so
eliminate the extra IPC.

BUG=404650
TEST=check trace that now GLRenderer::SwapBuffers only sends 2 IPC messages,
     i.e. AsyncFlush and Echo. And LatencyInfo tracking is still working.

Review URL: https://codereview.chromium.org/564903002

Cr-Commit-Position: refs/heads/master@{#294895}