[DevTools] Integrate with global zoom factor passed from embedder.

Previously, DevTools frontend was not aware of external zoom factor.
For example, setting global zoom to 150% and hitting Ctrl+ in DevTools
would make frontend smaller (110%) instead of enlarging it.
Also, this interacted badly with overlaying contents, where we have to
adjust split size to zoom.

Now, we use combined zoom factor, which is a multiplication of external
zoom and internal DevTools zoom level.

BUG=331461

Review URL: https://codereview.chromium.org/133743002

git-svn-id: svn://svn.chromium.org/blink/trunk@164984 bbb929c8-8fbe-4397-9dbb-9b2b20218538