Prefer opening PDF downloads in the browser.

PDFs in particular are safer to open in the browser. This patch changes
the handling of downloads to open such files in the browser by default
instead of the system handler for the file type. A "Open with system
handler" menu item will be available so that users can still use the
system application if needed.

The determination that a file is safer to handle in the browser is done
as follows:

a) DownloadTargetDeterminer determines whether the MIME type
   corresponding to the target filename of the download is one which is
   handled by the renderer or one that is handled by a sandboxed pepper
   plugin. If so, then the file is considered safely handled by the
   browser.

b) ChromeDownloadManagerDelegate determines whether opening in the
   browser is preferred for the file type assuming the browser is able
   to handle it safely. Currently this is true for .pdf files.

Opening behavior for a download will default to opening in the browser
if both results from a) and b) are true.

BUG=148492

TEST=(1) Make sure Chrome PDF Viewer is enabled in chrome://plugins.
     (2) Download a .pdf file.
     (3) Download shelf context menu should show "Open" and "Open with
         system handler" options.
     (4) "Open" as well as clicking on the shelf icon and clicking on
         the download in chrome://downloads should all result in opening
         the file within the browser.
     (5) "Open with system handler" should open the .pdf with the
         application that's set up as the default handler for .pdf files
         on the users' machine.

Review URL: https://codereview.chromium.org/55063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233460 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233460
Add test-info for media_perftests.

BUG=310837

Review URL: https://codereview.chromium.org/59573004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233461 0039d316-1c4b-4281-b951-d872f2087c98
Don't add keep-alives for apps after the app keep-alive service has shut down.

BUG=314729

Review URL: https://codereview.chromium.org/60193005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233462 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233462