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This is a reland of f5bbb138

Original change's description:
> Add SubmitFrameMissing mojo call for WebVR/WebXR
> 
> Goal is that we get a clean lifecycle for a functioning WebVR/WebXR
> presentation render loop. It's started by a presenting SendVSync, calls
> GetVSync to schedule the next frame, and is ended by a SubmitFrame
> call. If there was nothing drawn, it uses SubmitFrameMissing instead of the
> usual SubmitFrame/SubmitFrameWithTextureHandle.
> 
> In WebVR 1.1, submitFrame is a JS call, and the app can exit its animation
> loop without calling it. WebXR had an analogous feature where SubmitFrame
> was skipped if the framebuffer wasn't touched by drawing calls. This
> made it hard to tell for the device side if a frame is done or not.
> 
> WebVR 1.1 worked around this by deferring a GetVSync call until after
> SubmitFrame, but this was complex:
> 
> -  // The logic here is a bit subtle. We get called from one of the following
> -  // four contexts:
> -  //
> -  // (a) from requestAnimationFrame if outside an animating context (i.e. the
> -  //     first rAF call from inside a getVRDisplays() promise)
> -  //
> -  // (b) from requestAnimationFrame in an animating context if the JS code
> -  //     calls rAF after submitFrame.
> -  //
> -  // (c) from submitFrame if that is called after rAF.
> -  //
> -  // (d) from ProcessScheduledAnimations if a rAF callback finishes without
> -  //     submitting a frame.
> -  //
> -  // These cases are mutually exclusive which prevents duplicate GetVSync
> -  // calls. Case (a) only applies outside an animating context
> -  // (in_animation_frame_ is false), and (b,c,d) all require an animating
> -  // context. While in an animating context, submitFrame is called either
> -  // before rAF (b), after rAF (c), or not at all (d). If rAF isn't called at
> -  // all, there won't be future frames.
> 
> This CL removes those special cases and just always calls RequestVSync from
> requestAnimationFrame, collapsing cases (a) and (b) into an unconditional call.
> Cases (c) and (d) are now no longer needed and removed.
> 
> The layout tests now check the SubmitFrameMissing call count.
> 
> Also added a check for a WebXR exclusive session ending in the middle of a
> frame. (The layout tests revealed this since OnEndFrame no longer exits early
> on a clean framebuffer.)
> 
> Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
> Change-Id: I5722097d421ca9448760e696ea379895a1320199
> Reviewed-on: https://chromium-review.googlesource.com/996614
> Reviewed-by: Ian Vollick <vollick@chromium.org>
> Reviewed-by: Brandon Jones <bajones@chromium.org>
> Reviewed-by: Martin Barbella <mbarbella@chromium.org>
> Reviewed-by: Bill Orr <billorr@chromium.org>
> Commit-Queue: Klaus Weidner <klausw@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#548518}

Change-Id: I14b7ac8fc661cfb91a8be2b959d27091007602ee
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Reviewed-on: https://chromium-review.googlesource.com/998079Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Martin Barbella <mbarbella@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548770}