Add a MockAsyncProxyResolverFactory and update some tests to use it.

This CL:
- adds a mock implementation of ProxyResolverFactory;
- changes the ProxyService tests to use MockAsyncProxyResolverFactory
  instead of using a LegacyProxyResolverFactor; and
- removes ForwardingProxyResolverFactory and updates the tests that
  used it.

BUG=467403

Review URL: https://codereview.chromium.org/1102463002

Cr-Commit-Position: refs/heads/master@{#327431}
9 files changed