Enable the task manager to be put into immersive fullscreen

BUG=307622
TEST=AcceleratorCommandsBrowserTest.ToggleFullscreen
Manual
1) Enable "Enable immersive fullscreen for non browser windows" in chrome://flags
2) Open the task manager
3) With the task manager having focus, hit <F4>. Check that the window goes fullscreen
4) Hover the mouse at the top of the screen. Check that the window controls slide onscreen
5) Hit <F4> again. Check that the window exits fullscreen.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=239403

R=benwells@chromium.org, oshima@chromium.org

Review URL: https://codereview.chromium.org/84743003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239419 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239419