Add origin option for injected style sheets

crrev.com/c/641294 replaced injected author style sheets with user style
sheets. There are, however, legitimate use cases for the former. For
one, specificity does not work across origin boundaries; in such cases
extension developers may be forced to use "!important" as a hack.

This patch reintroduces the author origin for injected style sheets and
makes it the default by bringing back the relevant code deleted in
crrev.com/c/641294

WebDocument::InsertStyleSheet now takes an optional second argument of
type WebDocument::CSSOrigin

BUG=632009

Change-Id: Ia6e7018411faf47411aabfb82e39158650ccedb4
Reviewed-on: https://chromium-review.googlesource.com/765642
Reviewed-by: nainar <nainar@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: nainar <nainar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518913}
8 files changed