Modify PENDING_ADD_FAILED failure reason

As per the current code flow, PENDING_ADD_FAILED failure reason can
be reported when:
1) an extension with same extension_id is already installed from a
higher priority source (Manifest::Location) but is corrupted. This is
then not reported in the metrics as the extension is already installed.
2) extension is already in the pending extension list from a higher
priority source (Manifest::Location).

Case #2 should not be reported as PENDING_ADD_FAILED failure for the
force installed extension.

Bug: b:162855924
Change-Id: I1f91a6d5ae97c7f75fc69584d9ba0ed9516e6d79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379859


Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Oleg Davydov <burunduk@chromium.org>
Reviewed-by: Swapnil Gupta <swapnilgupta@google.com>
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Cr-Commit-Position: refs/heads/master@{#805666}