WebView: add assertion messages

No change to logic, this just adds assertion messages to help with
debugging. I came across this when investigating
https://crbug.com/1006953.

Bug: 123022731
Test: atest CtsWebkitTestCases:WebViewClientTest#testOnSafeBrowsingMalwareCode
Change-Id: Id4f44d65d8b53abf9d8142a600e86a7ee0bec94a
diff --git a/tests/tests/webkit/src/android/webkit/cts/WebViewClientTest.java b/tests/tests/webkit/src/android/webkit/cts/WebViewClientTest.java
index ec39a5c..8c3b252 100644
--- a/tests/tests/webkit/src/android/webkit/cts/WebViewClientTest.java
+++ b/tests/tests/webkit/src/android/webkit/cts/WebViewClientTest.java
@@ -902,8 +902,11 @@
         @Override
         public void onPageFinished(WebView view, String url) {
             super.onPageFinished(view, url);
-            assertTrue(mOnPageStartedCalled);
-            assertTrue(mOnLoadResourceCalled);
+            // TODO(ntfschr): propagate these exceptions to the instrumentation thread.
+            assertTrue("Expected onPageStarted to be called before onPageFinished",
+                    mOnPageStartedCalled);
+            assertTrue("Expected onLoadResource to be called before onPageFinished",
+                    mOnLoadResourceCalled);
             mOnPageFinishedCalled = true;
         }