Add support for refreshing contents of providers which don't support watchers.

Add support for refreshing contents of providers which don't support watchers.

This CL:
1. Adds a file_system_provider.watchable manifest option similar to the
   existing file_system_provider.configurable.
2. Exposes this flag to Files app.

TEST=browser_tests: *FileSystemProvider*Mount*
BUG=501864
Committed: https://crrev.com/b577564f300f2cd67cf29e3cd02229f1fd7dd610
Cr-Commit-Position: refs/heads/master@{#337564}

[mtomasz, 2015-07-02 08:51:42]

mtomasz@chromium.org changed reviewers:
+ benwells@chromium.org

[mtomasz, 2015-07-02 08:52:01]

@benwells: PTAL (at everything this time). Thanks.

[mtomasz, 2015-07-06 08:32:55]

@benwells: Friendly ping.

[benwells, 2015-07-07 00:18:53]

On 2015/07/06 08:32:55, mtomasz wrote:
> @benwells: Friendly ping.

There seem to be compile failures on the chromeos bots. What's up with that?

[mtomasz, 2015-07-07 02:30:00]

On 2015/07/07 00:18:53, benwells wrote:
> On 2015/07/06 08:32:55, mtomasz wrote:
> > @benwells: Friendly ping.
> 
> There seem to be compile failures on the chromeos bots. What's up with that?

@benwells: I fixed the tests. Most changes are mechanical. PTAL, thanks.

[benwells, 2015-07-07 03:38:53]

lgtm with the comment fixed.

https://codereview.chromium.org/1221093002/diff/20001/chrome/common/extension...
File chrome/common/extensions/api/file_manager_private.idl (right):

https://codereview.chromium.org/1221093002/diff/20001/chrome/common/extension...
chrome/common/extensions/api/file_manager_private.idl:537: // Whether supports
configuration dialog.
This comment is incorrect.

[mtomasz, 2015-07-07 03:44:47]

mtomasz@chromium.org changed reviewers:
+ fukino@chromium.org

[mtomasz, 2015-07-07 03:44:48]

@fukino: PTAL at third_party/closure_compiler/externs/file_manager_private.js.
Thanks.

https://codereview.chromium.org/1221093002/diff/20001/chrome/common/extension...
File chrome/common/extensions/api/file_manager_private.idl (right):

https://codereview.chromium.org/1221093002/diff/20001/chrome/common/extension...
chrome/common/extensions/api/file_manager_private.idl:537: // Whether supports
configuration dialog.
On 2015/07/07 03:38:53, benwells wrote:
> This comment is incorrect.

Done.

[fukino, 2015-07-07 04:14:08]

externs lgtm.

[mtomasz, 2015-07-07 04:14:22]

The CQ bit was checked by mtomasz@chromium.org

[mtomasz, 2015-07-07 04:14:23]

The patchset sent to the CQ was uploaded after l-g-t-m from
benwells@chromium.org
Link to the patchset: https://codereview.chromium.org/1221093002/#ps40001
(title: "Fixed.")

[commit-bot: I haz the power, 2015-07-07 04:14:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1221093002/40001

[mtomasz, 2015-07-07 04:42:12]

The CQ bit was checked by mtomasz@chromium.org

[mtomasz, 2015-07-07 04:42:12]

The patchset sent to the CQ was uploaded after l-g-t-m from
benwells@chromium.org, fukino@chromium.org
Link to the patchset: https://codereview.chromium.org/1221093002/#ps60001
(title: "Fixed tests.")

[commit-bot: I haz the power, 2015-07-07 04:42:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1221093002/60001

[commit-bot: I haz the power, 2015-07-07 06:07:54]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-07-07 06:09:06]

Patchset 4 (id:??) landed as
https://crrev.com/b577564f300f2cd67cf29e3cd02229f1fd7dd610
Cr-Commit-Position: refs/heads/master@{#337564}