Fortify autofill tests for WebView

Currently, we are testing whether certain callbacks are called / not
called in specific situations. This type of one-off tests are easily
isolatable, but it has its own limits - it is difficult to detect newly
added callbacks.

The new scheme audits all the calls into AutofillManager and ensures
that WebView autofill functionalities can work as expected.

Also, this CL refactors out some variables from TestAwAutofillManager
(renamed from AwAutofillManagerHelper) into AwAutofillTest and make
the test manager a thin wrapper.

Note: All the tests were run locally, as there is no trybot for O.

BUG=717658

Change-Id: Idacea03727f5f1cd56cd62dc1420dc74f48b04cf
Reviewed-on: https://chromium-review.googlesource.com/706379
Reviewed-by: Tao Bai <michaelbai@chromium.org>
Reviewed-by: Yoland Yan <yolandyan@chromium.org>
Commit-Queue: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509690}