Use the StyleResolverParentScope for ShadowRoot

In r170168 I made pushing the parent scopes lazy for Elements, but I
didn't make it lazy for ShadowRoot. This meant that you could trigger a
style recalc on a single element in a ShadowRoot and then we'd push the
scopes out of order, where we'd push all the ShadowRoots on the way down
and then push all the elements at the end.

I wasn't able to show any rendering errors, but by inspection it's wrong
and it appears it was making us go through the
ScopedStyleTree::resolveStyleCache code path more often than needed.

Review URL: https://codereview.chromium.org/216283003

git-svn-id: svn://svn.chromium.org/blink/trunk@170312 bbb929c8-8fbe-4397-9dbb-9b2b20218538
SRI: Add the 'integrity' attribute, and a SubresourceIntegrity runtime flag.

This patch adds a new flag to begin experimentation with the Subresource
Integrity specification[1], and begins adding the minimal amount of
functionality so we can test that the flag is working. That turns out to
be the 'integrity' attribute on various HTML elements that we'll use
later in order to do the actual verification.

Intent to Implement at [2].

[1]: http://w3c.github.io/webappsec/specs/subresourceintegrity/
[2]: https://groups.google.com/a/chromium.org/d/msg/blink-dev/hTDUpMk_TV8/t_rjlkKfgGgJ

BUG=355467

Review URL: https://codereview.chromium.org/208423011

git-svn-id: svn://svn.chromium.org/blink/trunk@170313 bbb929c8-8fbe-4397-9dbb-9b2b20218538