[Android WebViewLayoutTest] Improve error message in testWebViewIncludedStableInterfaces()

Improve error message when testWebViewIncludedStableInterfaces fails.
Also add a guideline on how to potentially fix it.

Previously there was some confusion on how to interpret the failure
(see crbug.com/859141).

Bug: 859141
Change-Id: I2ac1439ad164186142b16697e7378efb8a975629
Reviewed-on: https://chromium-review.googlesource.com/1127240
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576197}
diff --git a/android_webview/tools/system_webview_shell/layout_tests/src/org/chromium/webview_shell/test/WebViewLayoutTest.java b/android_webview/tools/system_webview_shell/layout_tests/src/org/chromium/webview_shell/test/WebViewLayoutTest.java
index adf4458..f9ad2c0 100644
--- a/android_webview/tools/system_webview_shell/layout_tests/src/org/chromium/webview_shell/test/WebViewLayoutTest.java
+++ b/android_webview/tools/system_webview_shell/layout_tests/src/org/chromium/webview_shell/test/WebViewLayoutTest.java
@@ -234,7 +234,13 @@
                 }
             }
         }
-        Assert.assertEquals("Missing webview interfaces found", "", missing.toString());
+
+        if (missing.length() > 0) {
+            Assert.fail("Android WebView is missing the following declared Blink interfaces: "
+                    + missing.toString()
+                    + ". Interfaces which are intentionally not exposed in WebView need to be"
+                    + " added to not-webview-exposed.txt");
+        }
     }
 
     @Test