No need to re-parse empty set of injected stylesheets.

setCompatibilityMode invalidates the injected stylesheet cache which means
styleResolverChanged() and an updating of the active stylesheets list. The
reason is that the injected stylesheets need to be re-parsed with the
correct strict/quirks mode. If the list of injected stylesheets is empty,
there are no stylesheets to reparse, so avoid styleResolverChanged().

This might not mean much in practice, depending on where the compat mode
may change, but the styleResolverChanged was noise while trying to analyze
the bug in question.

BUG=393480

Review URL: https://codereview.chromium.org/468903002

git-svn-id: svn://svn.chromium.org/blink/trunk@181125 bbb929c8-8fbe-4397-9dbb-9b2b20218538