MIPS: Skip test-heap/TestSizeOfRegExpCode

MIPS: Skip test-heap/TestSizeOfRegExpCode

After https://codereview.chromium.org/1180433003 the code-size exceeeds
the limit for MIPS. We have some optimizations in place for MIPS64 that
will be ported back to MIPS, and will investigate other code-size
improvements so we can re-enable this test.

BUG=

Review URL: https://codereview.chromium.org/1166353005

Cr-Commit-Position: refs/heads/master@{#28913}