[AF] Reduce no of AutofillWebDataBackend test implementations

This CL makes a cleanup in autofill sync unit-test, reducing code
duplication and getting rid of custom fakes that can be mocked in one
line. It also renames another existing fake to achieve more consistency
in naming (w.r.t. gmock).

This CL precedes a change in the AutofillWebDataBackend interface,
making it easier to implement.

Bug: 853688
Change-Id: Iaf9ec6151caa77e9b81297f50c061835443cfadd
Reviewed-on: https://chromium-review.googlesource.com/1235716
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592771}
9 files changed