Replace base::MockTimer with behavior-specific timers in //chrome

This CL replaces MockTimer with MockOneShotTimer or MockRepeatingTimer
as a preparation of OnceCallback support of base::Timer.
Also, this CL fixes unneeded usages of |retain_user_task| flag.

Bug: 850247
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:linux_vr;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I04ab3df4e85cdf28ca2585f455f79ab9336f585e
Reviewed-on: https://chromium-review.googlesource.com/1114560
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572016}