Fix a race in headless virtual time & compositor tests

A lack of explicit start time for frame caused real time clock to
be used for frame start, which may cause next frame to be dropped
if its (virtual) timestamp is earlier.

Bug: 909043, 843734
Change-Id: I4a909be2e1e4f69f0834a87ffdb3d402c450adfc
Reviewed-on: https://chromium-review.googlesource.com/c/1379181
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616925}
2 files changed