Set visible viewport value in screen emulation.

Saved the current value and restore it when emulation ends. During emulation
the visible viewport is set to the size of the emulated screen.

BUG=370035

Review URL: https://codereview.chromium.org/302193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274359 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed