WebGL conformance tests on the Mac 10.10 Retina Debug (AMD) bot are flakily hitting the DCHECK_LE in VideoRendererAlgorithm::OnLastFrameDropped.

Builds:
https://build.chromium.org/p/chromium.gpu/builders/Mac%2010.10%20Retina%20Debug%20%28AMD%29/builds/18165
https://build.chromium.org/p/chromium.gpu/builders/Mac%2010.10%20Retina%20Debug%20%28AMD%29/builds/18089
https://build.chromium.org/p/chromium.gpu/builders/Mac%2010.10%20Retina%20Debug%20%28AMD%29/builds/18081

Log snippet:
[13088:18691:1026/003749:FATAL:video_renderer_algorithm.cc(308)] Check failed: frame_queue_.front().drop_count <= frame_queue_.front().render_count (2 vs. 1)
0   libbase.dylib                       0x0000000119687dce _ZN4base5debug10StackTraceC2Ev + 30
1   libbase.dylib                       0x0000000119687e35 _ZN4base5debug10StackTraceC1Ev + 21
2   libbase.dylib                       0x0000000119720df0 _ZN7logging10LogMessageD2Ev + 80
3   libbase.dylib                       0x000000011971e9a5 _ZN7logging10LogMessageD1Ev + 21
4   libmedia.dylib                      0x000000012a93228e _ZN5media22VideoRendererAlgorithm18OnLastFrameDroppedEv + 734
5   libmedia.dylib                      0x000000012a9d13f7 _ZN5media17VideoRendererImpl14OnFrameDroppedEv + 103
6   libmedia_blink.dylib                0x0000000134eba049 _ZN5media20VideoFrameCompositor10CallRenderEN4base9TimeTicksES2_b + 713
7   libmedia_blink.dylib                0x0000000134eb9d41 _ZN5media20VideoFrameCompositor18UpdateCurrentFrameEN4base9TimeTicksES2_ + 321
8   libcc.dylib                         0x000000012b97b6c4 _ZN2cc28VideoFrameProviderClientImpl12OnBeginFrameERKNS_14BeginFrameArgsE + 1140
9   libcc.dylib                         0x000000012bc840b4 _ZN2cc17LayerTreeHostImpl18WillBeginImplFrameERKNS_14BeginFrameArgsE + 804