Fix the painting issue observed with windowed NPAPI Flash plugins when we switch...

Fix the painting issue observed with windowed NPAPI Flash plugins when we switch away from a tab which has an instance of
the plugin and switch back to it.

When we switch back to the tab with the plugin, the flash RVHWA window becomes visible. The code which runs in the visible notification
handler runs through the plugin cutout rects calculation, which basically tries to remove areas where the plugin should not paint from its
clip region. In this case the cutout rects comes out as empty and the clip region in the plugin window moves structure is NULL. We end up
setting the window region to NULL which basically causes the plugin window to become useless.

Fix is to not mess with the window region of the plugin if the cutout rects are empty.

BUG=341564
R=cpu@chromium.org, cpu
TBR=jam

Review URL: https://codereview.chromium.org/140753008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249763 0039d316-1c4b-4281-b951-d872f2087c98