Fix a regression which caused flash plugin installation to not work via the C...

Fix a regression which caused flash plugin installation to not work via the Chrome plugin installer. The regression was caused due to the
mouse hook proc getting installed assuming that the current plugin was flash and setting and releasing capture thus messing up the button
controls capture state causing it to not fire the WM_COMMAND messages for plugin installation and cancellation.

Fix is to not install the mouse hook for internal plugins. The other fix is to have the mouse hook be a noop if the underlying window is
not our plugin window.

Fixes bug http://code.google.com/p/chromium/issues/detail?id=28025

Bug=28025
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=48548

[darin (slow to review), 2010-05-28 22:50:44]

LGTM

[jam, 2010-05-28 23:09:50]

lgtm