Revert 152059 - Gradient overlay for constrained window

Revert 152059 - Gradient overlay for constrained window

As a part of the new constrained window look and feel we're changing the the solid gray overlay to a gradient overlay. The overlay fades in on show and fades out on hide.

Screenshots:
    light background: http://i.imgur.com/GJj6L.png
    dark background: http://i.imgur.com/s37da.png

BUG=140520
TEST=Run with --enable-frameless-constrained-dialogs.
  Navigate to http://www.imagemator.com/
  Click "Click to pick an image."

Review URL: https://chromiumcodereview.appspot.com/10855151

TBR=sail@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=152062