Splits CompileUnoptimizedCode into a non-main thread GenerateUnoptimizedCode and
a main thread FinalizeUnoptimizedCode phase. Adds Disallow<HeapAccess> scopes in
CompileUnoptimizedCode to ensure no access to the heap during this phase.

Also cleans up a few heap accesses in CompilationInfo's constructor to avoid
violating the disallowed heap access.

Currently we reallow heap access during asm.js compilation as a temporary
measure until the script streamer uses an off-heap script buffer.

BUG=v8:5203
TBR=titzer@chromium.org

Change-Id: I7f6140f19938a10a85f1cd89501812dd59dbf6d4
Reviewed-on: https://chromium-review.googlesource.com/605949
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47337}