Avoid passing isolate to CompileTopLevelOnBackgroundThread and instead
pass AccountingAllocator. This avoids storing isolate on BackgroundParsingTask

BUG=v8:5203

Change-Id: I1007858632ec6e2a7b4a7f3794eeb828b5707937
Reviewed-on: https://chromium-review.googlesource.com/753301Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49388}