Change the pageaction_by_content example to use declarativeContent.

The "Mappy" sample still provides an example of a content script searching for
text.

The new icon comes from http://en.wikipedia.org/wiki/File:Video_icon2.png, which
is public domain.

TEST=Install the extension, and visit http://www.html5rocks.com/en/tutorials/video/basics/. The page action icon should show up.
BUG=330359
NOTRY=true

Review URL: https://codereview.chromium.org/124603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243475 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243475