Worker: Stop running worker tests with BlinkGC::MainThreadHeapMode
Oilpan's per-thread heap mode is already shipped for workers and we no longer
have to run the tests on both BlinkGC::MainThreadHeapMode and
BlinkGC::PerThreadHeapMode.
BUG=
674400
Committed: 
https://crrev.com/d82e162920b55095efa40a8052e9539d2cf861bb
Cr-Commit-Position: refs/heads/master@{#438790}