Changes tab dragging code to ignore WS_EX_TRANSPARENT and WS_EX_LAYERED windows. Here's the comment I'm adding as to why:
Changes tab dragging code to ignore WS_EX_TRANSPARENT and
WS_EX_LAYERED windows. Here's the comment I'm adding as to why:

      // Mouse events fall through WS_EX_TRANSPARENT windows, so we ignore them.
      //
      // WS_EX_LAYERED is trickier. Apps like Switcher create a totally
      // transparent WS_EX_LAYERED window that is always on top. If we don't
      // ignore WS_EX_LAYERED windows and there are totally transparent
      // WS_EX_LAYERED windows then there are effectively holes on the screen
      // that the user can't reattach tabs to. So we ignore them. This is a bit
      // problematic in so far as WS_EX_LAYERED windows need not be totally
      // transparent in which case we treat chrome windows as not being obscured
      // when they really are, but this is better than not being able to
      // reattach tabs.

BUG=12580 10742
TEST=none

Review URL: http://codereview.chromium.org/3383004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@59562 0039d316-1c4b-4281-b951-d872f2087c98

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