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We current have a histogram for LayersUpdateTime but it is bucketed
by the layer count. This patch adds a new metric for the total
LayersUpdateTime that is not bucketed.

Bug: 825912
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie9d83f90abf529af97bc2393f8f3ad6dc2e3530d
Reviewed-on: https://chromium-review.googlesource.com/981262Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546208}