Add ash-enable-immersive-all-windows command line flag to enable immersive fullscreen for non-frameless v2 apps. (The goal is to add support for immersive fullscreen for v1 apps and miscellaneous windows such as the task manager shortly) When the flag is set, <F4> will put the app into immersive fullscreen.

In immersive fullscreen, the window header (the title bar and window controls) slide out as an overlay over the web contents when the mouse is hovered at the top of the display.

chrome.app.window.current().isFullscreen() returns false for immersive fullscreen windows
Entering fullscreen via the app.window api or webkitRequestFullScreen() quits immersive fullscreen.

BUG=307622
R=oshima
TBR=benwells (Because this is relanding https://codereview.chromium.org/59043013/), sky (For adding VIEWS_EXPORT to ui/views/widget/widget_observer.h)
TEST=Manual, see steps below:
1) Install: https://chrome.google.com/webstore/detail/window-state-sample/hcbhfbnaaancmblfhdknlnojpafjohbi
2) Enable "Enable immersive fullscreen for non browser windows" in chrome://flags
3) Hit <F4>
4) Ensure that the "isFullscreen" checkbox in the app is checked
5) Ensure that the shelf light bar is visible (The shelf is not completely hidden)
6) Hover the mouse at the top of the screen. Ensure that the window header (maximize and close button slide down and overlay the web contents)
7) Move the mouse off of the header. Ensure that the window header slides off screen
8) Press the Fullscreen button in the app (It may be useful to set the "chrome.app.window actions delay" to zero seconds)
9) Ensure that the shelf is completely hidden.
10) Hover the mouse at the top of the screen. Ensure that the header slides out again.
11) Press <F4>
12) Ensure that the "isFullscreen" checkbox is unchecked
13) Ensure that the window header is visible and does not slide out

Review URL: https://codereview.chromium.org/76483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236258 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236258