GitLab

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