[Autofill Assistant] Initialize counter in ScriptTracker::ScriptTracker.

I had forgotten to initialize pending_precondition_check_count_; running the
component tests with asan highlighted that.

Bug: 882901
Change-Id: Ia9621e80946da39e3d6ecb8e8c98a09beb4a76e2
Reviewed-on: https://chromium-review.googlesource.com/1219709Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590417}

components/autofill_assistant/browser/script_tracker.cc

@@ -14,7 +14,10 @@ namespace autofill_assistant {
 
 ScriptTracker::ScriptTracker(ScriptExecutorDelegate* delegate,
                              ScriptTracker::Listener* listener)
-    : delegate_(delegate), listener_(listener), weak_ptr_factory_(this) {
+    : delegate_(delegate),
+      listener_(listener),
+      pending_precondition_check_count_(0),
+      weak_ptr_factory_(this) {
   DCHECK(delegate_);
   DCHECK(listener_);
 }