Allow using the matched property cache with inline styles in shadow trees

The check for isInShadowTree() came from https://trac.webkit.org/changeset/110188
but there's no clear reason for the check besides the FIXME and the code
was added between when the last patch was uploaded to the webkit bug and
when it was committed manually so there's no history about the tests that were
run.

I can't find any reason that media control trees wouldn't handle caching
properly, and if they do we should fix the media controls instead of making
inline styles slow for all ShadowRoots.

Review URL: https://codereview.chromium.org/68853016

git-svn-id: svn://svn.chromium.org/blink/trunk@161952 bbb929c8-8fbe-4397-9dbb-9b2b20218538