Add visibility check to [ActivityStatus]

Some Java code checks to see whether Chrome is in the foreground when
it should really be checking if the Application's top Activity is
visible for scenarios where a little popup partially covers Chrome.

Add a method to make the check more explicit, then change obvious places
that do checks on whether Chrome is running to use the new function.

Precursor to https://gerrit-int.chromium.org/#/c/45073/

BUG=308211
R=nyquist@chromium.org, tedchoc@chromium.org, yfriedman@chromium.org

Review URL: https://codereview.chromium.org/27509007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229172 0039d316-1c4b-4281-b951-d872f2087c98
Create landing page for tutorial/index and move the part1 under tutorial/. Fix all the links accordingly.

The future part2 of the tutorial will go into the same directory alongside part1.

Also tweak the landing page for reference/

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3634
R=binji@chromium.org

Review URL: https://codereview.chromium.org/27692003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229173 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229172
SVN changes up to revision 229173