Clean up app list webstore_provider_browsertest.
Record whether a query is pending and use it to decide when to wait
(previously, it would wait if ANY previous query was ever pending, which
"just happened" to work for the specific test cases).
Changed some ASSERT into EXPECT.
Use more descriptive special strings for HTTP errors, rather than the
exact same string as the query, which is confusing.
Document the test cases properly.
Reformat the source according to ClangFormat.
BUG=481837
Review URL: https://codereview.chromium.org/
1133543003
Cr-Commit-Position: refs/heads/master@{#329327}