Cleanup: Move kAboutScheme constant into content namespace.

This moves the last constant from chrome to content namespace and fixes
the John's TODO.

$ git grep "namespace chrome" content/

returns 0 entries.

BUG=None
TEST=None, no functional changes
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/191293009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256015 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256015
Bug fix: Combobox: Focus on clicking mouse button

This CL fixes the regression that Combobox doesn't get focused when
clicking a right button of a mouse. Before introducing
TransparentButton, Combobox can get focus by itself on the mouse button
events, but now, TranparentButton robs the combobox of the events.
This CL has TransparentButton::OnMousePressed help the combobox with
focusing it.

BUG=350811
TEST=manual

Review URL: https://codereview.chromium.org/189913014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256016 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256016