Web Replay: add support to encode/decode the unsigned long ctype.
Web Replay: add helper function to get a frame's index without its document.
PluginStreamLoader should have an identifier before registering.

It seems that before, a plugin loader was registered with the
DocumentLoader unconditionally before it was initalized.
Since ResourceLoader::init() can fail, there's a chance that
some plugin loaders were being uselessly registered.
Move createUniqueIdentifier() from ProgressTracker to NetworkProxy.
Change DocumentLoader's set of resource loaders to a map.

It is indexed based on the loader identifier, which is unique.
This will allow us to dig out a loader and call it on replay.
Web Replay: unify deterministic loaderId and resource load identifiers.

The network proxy used to number resource handles based on its own
counter which was memoized during replay. This patch gets rid of this
special-purpose counter and uses resource load identifiers instead. It
also makes createUniqueIdentifier() deterministic.
Web Replay: simulate network callbacks directly by calling ResourceLoader.

We still capture network callbacks using ResourceHandle shims, but on
replay we simulate the callbacks by digging a ResourceLoader out of
the active document loader and calling the correct callback. This
removes handle ids from callback input subclasses and adds a frameIndex.
Web Replay: remove NetworkProxy handle map, consolidate callback classes.

ResourceHandles are no longer stored or used during replay.

Variants of ResourceDidFail were folded into one class. The memoized
ResourceLoaderDestroyed was removed, since it never worked anyway.

Remove callbacks from Capturing ResourceHandleClient that don't implement
any capturing functionality.
Web Replay: don't let ResourceLoaders load during replay.

Also, don't make dummy ResourceHandles and clients.
Web Replay: remove unused NetworkProxy::expectsPageLoad() flag.
Web Replay: capture network callback inputs from inside SubresourceLoader.

Additionally, don't schedule network requests with NetworkProcess if replaying.
Web Replay: remove ResourceHandle-based capture shim, since it's no longer used.

Closes #405.
Closes #183.