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This is a reland of a752710d

Original change's description:
> Use shared memory to send images to X server
>
> When playing back a 1440p 60fps video without hardware acceleration, ~15% of
> frames would be dropped. With this change, I'm not getting any dropped frames.
>
> Background:
> The bottleneck is in SoftwareOutputDeviceX11::EndPaint, which (for 4K video) was
> taking ~0.06s per frame, which was limiting the output to ~15fps. It was taking
> so long because we were sending each frame to the X server over the wire. With
> the shared memory approach from the above CL, SoftwareOutputDeviceX11::EndPaint
> takes only ~0.00002s.
>
> Summary of changes:
> * Switch usage of XPutImage() to XShmPutImage() when possible.
>   * Shm segment is recycled across frames.
>   * Segment has a 1.5x growth policy to give windows room to resize without
>     having to reallocate for each frame.
>   * Segment has space for 2 frames. It was experimentally found that bumping
>     this up to 3 didn't give significant gains.
> * X server sends a completion event when XShmPutImage() is finished, so we
>   cannot recycle the memory until getting this event.
>   * Event processing happens on the GPU main thread, but processing needs to
>     happen on the compositor thread, so forwarding the event is necessary. This
>     is implemented in XShmImagePool.
>
> BUG=991633
>
> Change-Id: Ia7782cb9e21d0a93d3250c118e4ba76025354e40
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693737
> Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
> Reviewed-by: kylechar <kylechar@chromium.org>
> Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
> Reviewed-by: Kenneth Russell <kbr@chromium.org>
> Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#691321}

R=jorgelo
TBR=rjkroege,kbr,kylechar

Bug: 991633
Change-Id: Id0533ea8ba4de45ca2242ee979197798a818826d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775500Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693202}