*and* report all "virtual" wasm scripts right when the wasm script is
registered at the inspector.

WasmScript is a subtype of Script, with the cast checking that it is
actually a wasm script.
This layout makes it quite easy to implement functionality that is only
available for wasm scripts, and allows to later directly use the
WasmCompiledModule instead of the i::Script for backing the
debug::WasmScript. We might also add virtual methods to
provide different implementations for GetSourcePosition, Source and
others.

DisassembleWasmFunction now also becomes a method of this class instead
of a static function on the DebugInterface.

The WasmTranslation now uses the new WasmScript type instead of the
Script wrapper, and also registers all virtual wasm scripts immediately
when the wasm script is made public to the inspector (when the wasm
module is created).

R=yangguo@chromium.org,dgozman@chromium.org,titzer@chromium.org
BUG=chromium:613110,chromium:659715

Review-Url: https://codereview.chromium.org/2531163010
Cr-Commit-Position: refs/heads/master@{#41519}