Don't release capture when this window doesn't already have it

Don't release capture when this window doesn't already have it

- frame called ::SetCapture
- mouse down on menu button
- menu opens and calls ::SetCapture
- Windows notifies frame that it's lost capture
- The DesktopCaptureClient release its internal capture state, but
  also releases native capture
- which releases the menu's capture, not its capture.

This makes sure that the window that's releasing capture is really
the one that has it.

R=ben@chromium.org
BUG=232155
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=194682

[Ben Goodger (Google), 2013-04-17 20:08:54]

lgtm

[scottmg, 2013-04-17 21:18:36]

Committed patchset #1 manually as r194682 (presubmit successful).