Added tests for taskbar anchoring on Mac.

This required a small amount of refactoring to separate the testable
part from the non-testable calls to the system.

BUG=317944

Review URL: https://codereview.chromium.org/70383006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236384 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236384
[Toolbar] Refactoring: move ButtonDropDown, update image loading in Reload button.


ButtonDropDown is only used in the toolbar -- moved there to become the button base class. Update image loading in reload button to be a bit later to allow the reload button to get the ThemeProvider itself.

Review URL: https://codereview.chromium.org/74483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236385 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236385