FileAPI: Make ObfuscatedFileUtil to receive SandboxFSDelegate in its ctor

After introducing PluginPrivateFSB, each SandboxFSBDelegate and the backend has
its own ObfuscatedFileUtil, and the util should not depend on the delegate when
it is owned by the backend.

This change makes the ctor of the util to receive a pointer of the delegate and
to check its existence when accessing it.


BUG=286240
TEST=content_unittests --gtest_filter=ObfuscatedFileUtilTest.*

Review URL: https://codereview.chromium.org/42713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230985 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230985
Extract FileSelectionDialogsPolicyHandler into its own file.

Note that the tests have been refactored to avoid reliance on the
ConfigurationPolicyPrefStore.

BUG=271392
R=pneubeck@chromium.org

Review URL: https://codereview.chromium.org/40133003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230986 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230986