Make sure backing store pointer in handles get cleared after use in factory.

BUG=442710
LOG=n

Review URL: https://codereview.chromium.org/813023002

Cr-Commit-Position: refs/heads/master@{#25881}
Monomorphic and polymorphic ICs with cleared maps should not go megamorphic.

BUG=v8:3663
TEST=cctest/test-heap/MonomorphicStaysMonomorphicAfterGC
LOG=N

Review URL: https://codereview.chromium.org/816653002

Cr-Commit-Position: refs/heads/master@{#25882}
Temporarily disable mjsunit/debug-stepframe for GC stress until
crash is fixed.

BUG=v8:3776
LOG=N
TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/812283004

Cr-Commit-Position: refs/heads/master@{#25883}