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This patch changes the behavior of willstart- an didfinish- lifecycle
callbacks, as well as when the start of lifecycle tasks are run.

Previously, they would run on any call to the lifecycle update.
However, the intent of this is to document when the main frame
lifecycle runs (ie to PaintClean).

R=szager@chromium.org, chrishtr@chromium.org

Fixed: 1121965
Change-Id: I83bfa7142ca3fc9a94adbd0e5dfbe0245da3c9be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386222Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803637}