app_shell: Fix crash on chrome.runtime.getPackageDirectoryEntry for cros

app_shell: Fix crash on chrome.runtime.getPackageDirectoryEntry for cros

Because app_shell does not support code in //chrome it cannot register the
special chromeos::FileSystemBackend that handles kFileSystemTypeNativeLocal.
However, code in //storage in IsolatedFileSystemBackend implicitly assumes
that on Chrome OS kFileSystemTypeNativeLocal will be handled elsewhere.

This patch changes IsolatedFileSystemBackend to provide support for
kFileSystemTypeNativeLocal whenever the //storage embedder does not provide a
backend for it. This preserves existing behavior for non-app_shell platforms
and makes app_shell behave like Linux, which works fine.

A better solution would be to refactor chromeos::FileSystemBackend out of
//chrome/browser/chromeos, but that is blocked on componentization of
//chrome/browser/drive, see http://crbug.com/257943

BUG=447027
TEST=existing FileSystem unit and browser tests, manual test of app_shell (see bug for test app)

Committed: https://crrev.com/92d7e42458467aabf64b147e8b95d0f4d205fa3f
Cr-Commit-Position: refs/heads/master@{#310780}

[James Cook, 2015-01-08 23:43:11]

jamescook@chromium.org changed reviewers:
+ tzik@chromium.org

[James Cook, 2015-01-08 23:43:11]

tzik, PTAL

(go/appshell has information about app_shell if you need it.)

[tzik, 2015-01-09 02:24:44]

lgtm

[James Cook, 2015-01-09 15:03:56]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2015-01-09 15:04:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/838323003/20001

[commit-bot: I haz the power, 2015-01-09 16:43:38]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-09 16:45:08]

Patchset 2 (id:??) landed as
https://crrev.com/92d7e42458467aabf64b147e8b95d0f4d205fa3f
Cr-Commit-Position: refs/heads/master@{#310780}