Revert 233460 "Prefer opening PDF downloads in the browser."

Caused "PluginPrefsTest.UnifiedPepperFlashState" failure on "Linux Clang (dbg)"

BUG=316017

> 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

TBR=asanka@chromium.org

Review URL: https://codereview.chromium.org/61623006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233497 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233497