Introduce Runtime flags for /deep/ and ::shadow used in CSS dynamic profile

See https://bugs.chromium.org/p/chromium/issues/detail?id=489954#c115 for the context.

We'll disable these features at M63. If something is wrong in canary after
disabled, we can re-enable these features easily by flipping the flags, without worrying about
merge conflicts.

Regarding layout tests, we had already removed most of layout tests for /deep/ and ::shadow
used in CSS dynamic profile. It might be better to recover some of them, however, that might
not be worth doing because I am disabling these features after M62 branch cut, which will happen
soon.

Site owners can test their sites by adding
'--disable-blink-features=DeepCombinatorInCSSDynamicProfile' and/or
'--disable-blink-features=ShadowPseudoElementInCSSDynamicProfile' switches.

I verified these flags worked correctly with the reported sites in bug 489954.

Bug: 489954
Change-Id: Iea9d4b43576633ad7c1b3935e2576777dcae9826
Reviewed-on: https://chromium-review.googlesource.com/618307
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495474}
11 files changed