[DevTools] Migrate device emulation to WebDeviceEmulationParams.

This allows to apply additional scale when leaving original
view size - to be used in responsive design fit to view mode.

BUG=327641

Review URL: https://codereview.chromium.org/220943003

git-svn-id: svn://svn.chromium.org/blink/trunk@170788 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Let the overlay playback button fall back to the activation behavior

Since the button is only visible when togglePlayStateWillPlay(), just
doing nothing and falling back to the default activation behavior will
do the right thing.

BUG=354746
TEST=LayoutTests/media/activation-behavior-shadow.html

Review URL: https://codereview.chromium.org/215323003

git-svn-id: svn://svn.chromium.org/blink/trunk@170789 bbb929c8-8fbe-4397-9dbb-9b2b20218538