Implement Platform::createWaitableEvent(): chromium side

blink side: https://codereview.chromium.org/138303002/

BUG=334263

Review URL: https://codereview.chromium.org/133493003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245858 0039d316-1c4b-4281-b951-d872f2087c98
Switch off always-on-top bit when the app window overlaps the taskbar

To prevent apps from covering and spoofing the Windows taskbar, the
always-on-top bit will be silently switched off when there is any
overlap between the app window and the Windows taskbar. The bit is
reinstated when the window moves away from the taskbar.

BUG=320107
TEST=Use the Window State sample Chrome app to test manually.
On Win 7, verify that when an app window with always-on-top enabled
cannot be above the taskbar. See bug (comment #30) for known issues.

Review URL: https://codereview.chromium.org/137533008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245859 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245859