Use offline help app URL for some links in Files.app.

Along the way, renamed several constants to match the behavior.

BUG=161422

Review URL: https://codereview.chromium.org/103293005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238647 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238647
We want the Applications Menu shortcut paths to be managed by shell_util.cc as
this will allow the uninstaller to clean up shortcuts. BrowserDistribution now
provides the name of a distribution-specific Chrome app subfolder, and
ShellUtil's utility functions can now create, update, and remove shortcuts
therein.

This CL introduces some structural changes:

-ShellIntegration::ShortcutLocations now uses an enum to specify the
Applications Menu location rather than a string. The enum consists of the root
of the applications menu, the "Google Chrome" subdirectory and the "Chrome
Apps" subdirectory.

-ShellUtil::ShortcutLocation has the equivalent locations which are implemented
by ShellUtil::GetShortcutPath().

-BrowserDistribution::GetStartMenuShortcutSubfolder() now
supports SUBFOLDER_APPS.

This CL can be supplementally tested by running setup_unittests.

This began a rework of https://codereview.chromium.org/13940006/ for gab@'s
post-commit comments.

BUG=233434,238895
TEST=setup_unittests.exe, installer_util_unittests.exe

Review URL: https://codereview.chromium.org/13864015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238648 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238648