[Extensions Bindings] Make APIBindingsSystemTest reusable

APIBindingsSystemTest allows the construction of a full
APIBindingsSystem (including request/event handlers, constructed API
bindings, type reference map, etc), but with custom-made APIs (from a
provided JSON spec). This can be useful in testing, since it means we
can supply our own bindings and don't have to rely on the "real" ones
that the NativeExtensionBindingsSystemUnittest uses.

Split api_bindings_system_unittest.cc into a .cc and .h so that we
can reuse the test class elsewhere.

BUG=653596

Review-Url: https://codereview.chromium.org/2763493003
Cr-Commit-Position: refs/heads/master@{#458262}
3 files changed