Defer ARC file system operations while ARC is booting.

Defer ARC file system operations while ARC is booting.

An additional abstraction layer, ArcFileSystemOperationRunner, is inserted
above mojom::FileSystemInstance. Its production implementation is
ArcDeferredFileSystemOperationRunner, which defers file system operations
while ARC is booting.

This provides better UX when the user attempts to perform file operations
while ARC is booting. For example:

- Media views are mounted in Files app soon after the user logs into
  the system. If the user attempts to open media views before ARC boots,
  a spinner is shown until file system gets ready because ReadDirectory
  operations are deferred.
- When an Android content URL is opened soon after the user logs into
  the system (because the user opened the tab before they logged out for
  instance), the tab keeps loading until ARC boot finishes, instead of
  failing immediately.

This patch also changes following points for better media view UX:

- Mount/unmount media view volumes according to ARC opt-in status, not ARC
  file system instance availability.
- Add a short circuit in ArcDocumentsProviderRoot to return a metadata for
  root directories. Otherwise Files app fails to update the file system list
  (the left pane of UI) until deferred operations finish.

BUG=chromium:682577
TEST=unit_tests
TEST=Media views are mounted immediately, and a spinner is shown if it is opened.

Review-Url: https://codereview.chromium.org/2637163002
Cr-Commit-Position: refs/heads/master@{#445657}
(cherry picked from commit d280ab1a052245d5bcacf63a4478fc3c0f085899)

Review-Url: https://codereview.chromium.org/2658573002 .
Cr-Commit-Position: refs/branch-heads/2987@{#82}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed: https://chromium.googlesource.com/chromium/src/+/6cbdfc5ed5ae256f03033c0bf7c80bc398d6929c

[Shuhei Takahashi, 2017-01-25 05:49:59]

Description was changed from

==========
Defer ARC file system operations while ARC is booting.

An additional abstraction layer, ArcFileSystemOperationRunner, is inserted
above mojom::FileSystemInstance. Its production implementation is
ArcDeferredFileSystemOperationRunner, which defers file system operations
while ARC is booting.

This provides better UX when the user attempts to perform file operations
while ARC is booting. For example:

- Media views are mounted in Files app soon after the user logs into
  the system. If the user attempts to open media views before ARC boots,
  a spinner is shown until file system gets ready because ReadDirectory
  operations are deferred.
- When an Android content URL is opened soon after the user logs into
  the system (because the user opened the tab before they logged out for
  instance), the tab keeps loading until ARC boot finishes, instead of
  failing immediately.

This patch also changes following points for better media view UX:

- Mount/unmount media view volumes according to ARC opt-in status, not ARC
  file system instance availability.
- Add a short circuit in ArcDocumentsProviderRoot to return a metadata for
  root directories. Otherwise Files app fails to update the file system list
  (the left pane of UI) until deferred operations finish.

BUG=chromium:682577
TEST=unit_tests
TEST=Media views are mounted immediately, and a spinner is shown if it is
opened.

Review-Url: https://codereview.chromium.org/2637163002
Cr-Commit-Position: refs/heads/master@{#445657}
(cherry picked from commit d280ab1a052245d5bcacf63a4478fc3c0f085899)
==========

to

==========
Defer ARC file system operations while ARC is booting.

An additional abstraction layer, ArcFileSystemOperationRunner, is inserted
above mojom::FileSystemInstance. Its production implementation is
ArcDeferredFileSystemOperationRunner, which defers file system operations
while ARC is booting.

This provides better UX when the user attempts to perform file operations
while ARC is booting. For example:

- Media views are mounted in Files app soon after the user logs into
  the system. If the user attempts to open media views before ARC boots,
  a spinner is shown until file system gets ready because ReadDirectory
  operations are deferred.
- When an Android content URL is opened soon after the user logs into
  the system (because the user opened the tab before they logged out for
  instance), the tab keeps loading until ARC boot finishes, instead of
  failing immediately.

This patch also changes following points for better media view UX:

- Mount/unmount media view volumes according to ARC opt-in status, not ARC
  file system instance availability.
- Add a short circuit in ArcDocumentsProviderRoot to return a metadata for
  root directories. Otherwise Files app fails to update the file system list
  (the left pane of UI) until deferred operations finish.

BUG=chromium:682577
TEST=unit_tests
TEST=Media views are mounted immediately, and a spinner is shown if it is
opened.

Review-Url: https://codereview.chromium.org/2637163002
Cr-Commit-Position: refs/heads/master@{#445657}
(cherry picked from commit d280ab1a052245d5bcacf63a4478fc3c0f085899)

Review-Url: https://codereview.chromium.org/2658573002 .
Cr-Commit-Position: refs/branch-heads/2987@{#82}
Cr-Branched-From:
ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed:
https://chromium.googlesource.com/chromium/src/+/6cbdfc5ed5ae256f03033c0bf7c8...
==========

[Shuhei Takahashi, 2017-01-25 05:50:02]

Committed patchset #1 (id:1) manually as
6cbdfc5ed5ae256f03033c0bf7c80bc398d6929c.