GitLab

Refactor OpenFileToRead() to prepare for MTP support in ARCVM.

1. In OpenFileToRead(), the main work for MTP flow is refactored into
GenerateVirtualFileId() and OpenFileById().
2. Added 2 new FileSystemHost mojo call: GetVirtualFileId() and
HandleIdReleased().

BUG=b:158606367
TEST=As a regression test, in ARC++, a file can still be opened
with an Android app via MTP successfully.

Change-Id: Iea6cedd37903f94138428016b663e225d05138dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444757Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Cherie Cheung <cherieccy@google.com>
Cr-Commit-Position: refs/heads/master@{#821663}