Add trace event for identifying style invalidations

This adds an off-by-default "style.debug" trace
which records all style invalidations larger than
a certain threshold (currently 100) in the trace viewer
with JS stacks.

Hopefully this will help authors to better understand what part of their page is causing style invalidations.

This worked well for Baker when debugging his app.
Hopefully it will work well for others and we can add
something more web-dev-friendly in devtools
which is tracked by crbug.com/325900.

BUG=316388
R=nduca@chromium.org

Review URL: https://codereview.chromium.org/104983004

git-svn-id: svn://svn.chromium.org/blink/trunk@163916 bbb929c8-8fbe-4397-9dbb-9b2b20218538