GitLab

 - use let and const and add doc comment annotations.
 - update LoadTimeData fake to include DRIVE_OFFLINE_COLLECTION_LABEL
   and DRIVE_SHARED_WITH_ME_COLLECTION_LABEL to avoid JS exceptions
   about them being missing / undefined when running the unittest.
 - rm duplicate NOT_MOUNTED_SINGLE_PROVIDING_EXTENSION definition.
 - use VolumeManagerCommon.Source types rather than bare strings.
 - create & install mock chrome.fileManagerPrivate.
 - create & install mock VolumeManager.
 - run JS formatter over the code.

Bug: 905933
Change-Id: I7dabbb8ff72f79d290eaeffcb941280715f93660
Reviewed-on: https://chromium-review.googlesource.com/c/1343397
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609487}