Bug 797615 - Cosmetic cleanup for the getDisplayPort JNI wrapper. r=Cwiiis
Bug 797615 - Guard against isBrowserContentDocumentDisplayed returning true prematurely. r=Cwiiis

Prior to this change, isBrowserContentDocumentDisplayed returned false
from the time that the isFirstPaint flag was set in layout to the time
that layout handed off the rendered document to the compositor. However
the way the function is used meant that it needs to return false until
the compositor actually composites the "first-paint" rendering,
otherwise other events can sneak in and run before the compositor. This
patch moves the tracking for the flag into GeckoLayerClient so that it
can be queried and modified synchronously from both the Gecko thread in
browser.js and the compositor thread in setFirstPaintViewport.