It looks like the problem was not making the OS Window visible.
It turned out the previous version of the test succeeded only
when I ran angle_end2end_tests.exe with
--gtest_filter=EGLSyncControlTest.SyncValuesTest and failed
when I ran all tests, especially with --single-process-tests.

I tested the modified version of the test which repeated the
measurement 50 times and couldn't get a reliable test execution
until I figured out that back buffers were queueing and not
swapping successfully with the window being invisible.

This version works reliably when running all angle_end2end_tests.exe
on my workstation and on a test laptop, both running Windows 10,
with or without --single-process-tests.

BUG=angleproject:1402

Change-Id: I97764a86b20119cc586ea2a4fd1d94558c4e737e
Reviewed-on: https://chromium-review.googlesource.com/397739Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Stanislav Chiknavaryan <stanisc@chromium.org>