Mac: Don't use a dummy NSView as a gfx::AcceleratedWidget
Originally, each ui::Compositor did actually render into a NSView
on Mac, at which point it did make sense to use a NSView as the
gfx::AcceleratedWidget for the ui::Compositor. This has since
been changed due to stability and memory and performance issues.
Now, make the gfx::AcceleratedWidget be a key in a map that is
used to look up the corresponding BrowserCompositorViewMacInternals.
There is a chance that lifetime issues related to the NSView scheme
were causing crashes by sending messages to dead objects.
BUG=394051
Review URL: https://codereview.chromium.org/
420143004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@286652 0039d316-1c4b-4281-b951-d872f2087c98