Fixed skipping tests via test expectations.

Fixed skipping tests via test expectations.

1) Implement skip test expectations using SharedPageState.CanRunOnBrowser.
This avoids doing implicit operations in Telemetry for these pages,
and replicates the earlier behavior when test expectations were
implemented in Telemetry.

2) Provide gpu_test_base.ValidatorBase.WillNavigateToPageInner and
override this in trace_test. This makes the overriding of Page and
PageTest's entry points more complete. Also provide
DidNavigateToPageInner.

3) Rename all of the "private" PageTests in content/test/gpu to public
names so discover.py can find them. This makes the unit tests more
complete, and found a bug in the Maps test where test expectations
would not have worked.

4) Expand the unit tests to cover the fixes above.

BUG=517232

Review URL: https://codereview.chromium.org/1275273002

Cr-Commit-Position: refs/heads/master@{#342502}