The WasmCodeManager held a list of all Isolates that use the
WasmEngine/WasmCodeManager (those two are 1:1).
Since we want to move all isolate-specific tasks (like code logging and
compilation callbacks) to the WasmEngine, this CL moves this management
from the WasmCodeManager to the WasmEngine. We now have a bidirectional
mapping from NativeModules to the Isolates that use them, and from an
Isolate to all the NativeModules it uses (n:n).
The IsolateData struct will be extended in follow-up CLs to hold things
like the ForegroundTaskRunner. The Isolate* in the NativeModule /
CompilationState will eventually be removed.

R=mstarzinger@chromium.org

Bug: v8:8689
Change-Id: Ic2c003c3949f73ce3264dd9dac96884a5c0b9896
Reviewed-on: https://chromium-review.googlesource.com/c/1433793
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59092}