During GC multiple threads can request icache flush when evacuating
code space in parallel.

Simulator::FlushICache updates Isolate::simulator_icache hashmap,
which leads to a race.

This patch adds a lock for simulator_icache.

BUG=

Review-Url: https://codereview.chromium.org/2338793002
Cr-Commit-Position: refs/heads/master@{#39386}