[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}
1 file changed