[fsp] Unmount file systems when the providing extension gets unloaded.

Previously, if the extension got unloaded, provided the file system would still
be active, but not responsive. This CL listens for unload events, and removes
all of the provided file systems associated with the unloaded extension.

TEST=unit_tests:*FileSystemProviderService/UnmountFileSystem_OnExtensionUnload*
BUG=248427

Review URL: https://codereview.chromium.org/213123008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@264967 0039d316-1c4b-4281-b951-d872f2087c98
5 files changed