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- Remove "get" prefix us unnecessary, because method returns value
  directly
- Return error from ChromeEarlGreyAppInterface and assert that error is
  nil inside ChromeEarlGrey. This way the test will fail and print the
  error onstead of relying on caller to do the right thing.

Bug: None
Change-Id: I1d52347ae48b998b29e6a59d037f3adbbf5025b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521840Reviewed-by: Guillaume Jenkins <gujen@google.com>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825386}