[feature_engagement] Guard retrieving Tracker with null profile

This CL builds on top of http://crrev.com/c/3818263.

Before this CL, if the `profile` passed in to
TrackerFactory#getTrackerForProfile is null, we would return null.
This would mean that if the Tracker is immediately in use, we'd
identify where. Otherwise, we might never see a crash (if the Tracker
is null-checked or otherwise not in use), or see a crash much later
if the Tracker is cached.

After this CL, we will throw an exception at the Tracker retrieval
instead. This will guarantee that we will be able to identify anyone
invoking this incorrectly.

(cherry picked from commit ba8df59)

Bug: 1346710
Change-Id: I5af1534df241a513e65481d45a6860ab6d5853c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3817568
Commit-Queue: Siddhartha S <ssid@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1032834}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3816903
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Krishna Govind <govind@chromium.org>
Cr-Commit-Position: refs/branch-heads/5228@{#4}
Cr-Branched-From: 24425f2-refs/heads/main@{#1032780}

chrome/browser/feature_engagement/java/src/org/chromium/chrome/browser/feature_engagement/TrackerFactory.java

@@ -29,7 +29,9 @@ private TrackerFactory() {}
      */
     public static Tracker getTrackerForProfile(Profile profile) {
         if (sTestTracker != null) return sTestTracker;
-        if (profile == null) return null;
+        if (profile == null) {
+            throw new IllegalArgumentException("Profile is required for retrieving tracker.");
+        }
 
         return TrackerFactoryJni.get().getTrackerForProfile(profile);
     }