From r433356: https://build.chromium.org/p/chromium.memory.full/builders/Linux%20MSan%20Tests/builds/3374

==1==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x1ccbb4d8 in naturalSize content/renderer/media/webmediaplayer_ms.cc:377:51
    #1 0x1a10b9c8 in calculateIntrinsicSize third_party/WebKit/Source/core/layout/LayoutVideo.cpp:90:36
    #2 0x1a10c42e in updateIntrinsicSize third_party/WebKit/Source/core/layout/LayoutVideo.cpp:57:21
    #3 0x1a10cfae in updatePlayer third_party/WebKit/Source/core/layout/LayoutVideo.cpp:152:3

  Uninitialized value was stored to memory at
    #0 0x1ccbe5c5 in OnFirstFrameReceived content/renderer/media/webmediaplayer_ms.cc:607:19
    #1 0xac3e400 in Run base/callback.h:47:12
    #2 0xac3e400 in RunTask base/debug/task_annotator.cc:52:0

  Uninitialized value was stored to memory at
    #0 0x1ccbfd73 in __tuple_leaf\u003Cmedia::VideoRotation &, void> buildtools/third_party/libc++/trunk/include/tuple:225:15
    ...
    #7 0x1ccbfd73 in OnVideoFrame content/renderer/media/webmediaplayer_ms.cc:91:0

  Uninitialized value was created by an allocation of 'video_rotation' in the stack frame of function '_ZN7content16WebMediaPlayerMS14FrameDeliverer12OnVideoFrameE13scoped_refptrIN5media10VideoFrameEE'
    #0 0x1ccbece0 in OnVideoFrame content/renderer/media/webmediaplayer_ms.cc:66:0

I'll comment on the CL and point out where this went wrong.