Extract tree root responsibility out of HTMLImportsController.

The HTMLImportsController wishes to become more of an Actor-style object,
and it being a root of the import tree leads to ambiguities in responsibilities.
So we should split these responsibilities cleanly.

R=morrita
BUG=

Review URL: https://codereview.chromium.org/301803002

git-svn-id: svn://svn.chromium.org/blink/trunk@175108 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[DevTools] Cleanup inspector overlay and make it work with virtual viewport.

This also fixes "inspect element" for virtual viewport.

BUG=371843

Review URL: https://codereview.chromium.org/309473002

git-svn-id: svn://svn.chromium.org/blink/trunk@175109 bbb929c8-8fbe-4397-9dbb-9b2b20218538