Custom Tabs: improve test failure message CustomTabActivityTest.

Change-Id: Idfb16f09178adcd7399601d035870b51bacef84c
Reviewed-on: https://chromium-review.googlesource.com/962143Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543070}

chrome/android/javatests/src/org/chromium/chrome/browser/customtabs/CustomTabActivityTest.java

@@ -2867,7 +2867,11 @@ public class CustomTabActivityTest {
                 e.printStackTrace();
                 return false;
             }
-            return TextUtils.equals(mExpected, value);
+            boolean isSatisfied = TextUtils.equals(mExpected, value);
+            if (!isSatisfied) {
+              updateFailureReason("Page element is " + value + " instead of expected " + mExpected);
+            }
+            return isSatisfied;
         }
     }