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* AppActivity is now base class. There will be one for chrome
* Moved content_activity_factory.h from public to impl as ther eis no need to expose header.
* Added CreateAppActivity that takes apps::Window. Chrome
 impl will use this to create an activity. This simply returns NULL for app_shell version.

BUG=397167
R=mukai@chromium.org
TBR=jamescook@chromium.org
TEST=no functional change. all tests must pass.

Review URL: https://codereview.chromium.org/505273002

Cr-Commit-Position: refs/heads/master@{#292038}