Ensure window border icons repaint when DWM is enabled

When the user movers the cursor over the minimize/maximize/close
buttons, DWM paints them in a hover state. If the user subsequently
moves the cursor out of the window, DWM was not repainting the icons
with the unhovered state because Chrome ate the WM_NCMOUSELEAVE
message.This change notifies DWM of the exit.

TEST=Unmaximize Chrome. Hover Red-X at top right. Fling mouse upward.


Review-Url: https://codereview.chromium.org/2244263002
Cr-Commit-Position: refs/heads/master@{#412541}
1 file changed