Make some methods on V8DebuggerScript virtual and provide the
implementations ActualScript for scripts which are backed by scripts on
V8's side, and WasmVirtualScript for wasm scripts.

The added test case ensures that we at least don't crash on the attempt
to get breakable locations for wasm "scripts", which we did previously.
Returning a reasonable result for wasm will be implemented in a
follow-up commit.

R=yangguo@chromium.org, jgruber@chromium.org
BUG=chromium:667767,chromium:613110

Review-Url: https://codereview.chromium.org/2532433003
Cr-Commit-Position: refs/heads/master@{#41527}