This CL removes the JSGraph from WasmGraphBuilder and uses MachineGraph,
which is independent of the isolate, instead. In addition to using
the machine graph in the WasmGraphBuilder, this CL splits off a subclass
for compiling wrappers that does have a JSGraph and encapsulates it in
the .cc file. This makes the separation of WASM function graphs and WASM
wrapper graphs more explicit.

R=mstarzinger@chromium.org
CC=ahaas@chromium.org
BUG=v8:7721

Change-Id: I3c190baef2084919d22a9a89a8c9f11d2ddcf3d0
Reviewed-on: https://chromium-review.googlesource.com/1050266
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53086}