Rename ChromiumTestShell classes to ChromeShell.

Renaming ChromiumTestShell* classes to ChromeShell*.
This CL depends on https://chromiumcodereview.appspot.com/184493006/

BUG=348137
NOTRY=true
TBR=pfeldman,tonyg,craigdh

Review URL: https://codereview.chromium.org/184993005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255073 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255073