New DevTools extension overview, plus updates to inspectedWindow intro.

Review URL: https://codereview.chromium.org/23464098

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228820 0039d316-1c4b-4281-b951-d872f2087c98
Linux Aura: Fixed all windows with non-rounded corners being invisible.

Previously, it would get the window mask for the window, and always
apply it, even if the mask was empty (as it is for rectangular windows),
which would result in a completely invisible window.

BUG=306911

Review URL: https://codereview.chromium.org/27302002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228821 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228820
SVN changes up to revision 228821