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This CL updates all remaining user of base::Timer to use its subclasses,
and renames base::Timer to base::TimerBase to ensure there's no user
anymore.

Bug: 850247
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:ios-simulator-full-configs;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;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I73d70cbbe338e17f5d7de34acd1c961ce05c4305
Reviewed-on: https://chromium-review.googlesource.com/1124200
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576447}