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This CL allows calling shelf's SetAlignment and GetAlignment from
private autotest JavaScript API.
Calls to Ash are done via Mojo.

These testing APIs, including older ones like Shelf AutoHide API, are
placed in the ShelfIntegrationTestApi.

It also removes the "ForTesting" suffix from AutoHide testing APIs,
since it is no longer needed. It is implicit from its interface name:
ShelfIntegrationTestApi

This is needed for some integration tests, in particular ARC++ Tast
test.

This CL also fixes two bugs related to reporting errors:
 - Error() was using format string before, which was not supported
 - BrowserTest: setShelfBehavior() was reporting success,
   even after a failure

TEST=browser_tests --gtest_filter=AutotestPrivateApiTest.AutotestPrivate
     Tested manually from Tast test (WIP) on Nocturne
BUG=chromium:949837

Change-Id: I53813036b2adb9ae97c08a667f5fa13b715de3fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1554796
Auto-Submit: Ricardo Quesada <ricardoq@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ricardo Quesada <ricardoq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#655816}