viz: Remove unnecessary flags for presentation feedback.

Clients now always request presentation feedback for every frame (after
crrev.com/616913). So the following flags are no longer needed:

 . CompositorFrameMetadata.request_presentation_feedback
 . LayerTreeSettings.always_request_presentation_time
 . always-request-presentation-time command-line switch.

So remove these flags and associated code.

BUG=883592
TBR=danakj@ mechanical changes in chrome/browser, exo, content/renderer

Change-Id: I6c3032ea78d58fbd2f22ee5994c1def92605d162
Reviewed-on: https://chromium-review.googlesource.com/c/1379313
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618778}
26 files changed