Add trace event for updateActiveStyleSheets.
Add trace event for updateActiveStyleSheets.

Added for inspecting performance changes for async stylesheet update
and show how much time stylesheet update will account for when moved to
the updateLayoutTree part of the lifecycle.

Will not be called often enough to cause a performance issue in itself.

R=mstensho@opera.com
BUG=567021

Review URL: https://codereview.chromium.org/1843063005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384237}

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