As the code on the blink side sits down, we realize we don't need isolate arg
anymore. As the heap tracer is set per isolate, it can actually be confusing if
the isolate passed as argument is always the same as the isolate the heap tracer
was set for. Wdyt?

BUG=468240
LOG=no

Review URL: https://codereview.chromium.org/1900953003

Cr-Commit-Position: refs/heads/master@{#35620}