Allow stepping into Object.observe handlers.

BUG=chromium:432468
R=yangguo@chromium.org, adamk@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/739523002

Cr-Commit-Position: refs/heads/master@{#25423}
MIPS64: Add turbofan support for mips64.

TEST=
BUG=
R=danno@chromium.org, paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/732403002

Cr-Commit-Position: refs/heads/master@{#25424}
MIPS64: Prepare additonal code for turbofan landing.

TEST=
BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/735033002

Cr-Commit-Position: refs/heads/master@{#25425}
[turbofan]: delay ssa deconstruction in register allocator

BUG=

Review URL: https://codereview.chromium.org/738853002

Cr-Commit-Position: refs/heads/master@{#25426}