The isolate is mainly used for accounting purposes. As such, it
doesn't need a field in the WasmCodeManager, and cannot have one
if it is to be made isolate independent. Instead, pass the isolate
explicitly in the appropriate cases.

R=mstarzinger@chromium.org
BUG=v8:7424

Change-Id: I539c2b33692e57605a280530bd704ef25269ad0f
Reviewed-on: https://chromium-review.googlesource.com/1073412
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53676}