Qutoa: Fix divide-by-0 crash.

Add an guard to QuotaManager::MaybeRunStoragePressureCallback() for
the case when |total_space| is 0. Also add a TODO to figure out
how we want to handle this case over the long run.

Bug: 1059221
Change-Id: I9abba6cf21f810339fc6f115d9d484eaaed88abe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2093692Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Jarryd Goodman <jarrydg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748306}

storage/browser/quota/quota_manager.cc

@@ -1492,6 +1492,11 @@ void QuotaManager::MaybeRunStoragePressureCallback(const url::Origin& origin,
                                                    int64_t total_space,
                                                    int64_t available_space) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
+  // TODO(https://crbug.com/1059560): Figure out what 0 total_space means
+  // and how to handle the storage pressure callback in these cases.
+  if (total_space == 0)
+    return;
+
   if (!storage_pressure_callback_) {
     // Quota will hold onto a storage pressure notification if no storage
     // pressure callback is set.