Change shadow style's scope from Shadow Host to Shadow Root

No visible change, test expectation change is expected.

Scope of styles in shadow tree used to be its shadow host
for ease of checking the style against :host etc.

This caused code complexity for checking against boundary
breaking CSS selector rules (e.g. ::content, /deep/).
Changing the scope to its shadow root will simplify some
condition checks.

This is a step towards fixing issue 355674.

BUG=355674
TEST=pass all layout tests

Review URL: https://codereview.chromium.org/313303002

git-svn-id: svn://svn.chromium.org/blink/trunk@176304 bbb929c8-8fbe-4397-9dbb-9b2b20218538