Do fewer encoding checks in FlatStringReader used in the JSON stringifier.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/740673002

Cr-Commit-Position: refs/heads/master@{#25416}
PropertyDetails cleanup: NORMAL property type merged with FIELD.

First step towards replacing PropertyType with two enums: {DATA,ACCESSOR} x {CONST,WRITABLE}.

Review URL: https://codereview.chromium.org/733253004

Cr-Commit-Position: refs/heads/master@{#25417}
MIPS: Skip some tests on big-endian boards.

Skip one hard fail, and two frequent flakes while we investigate.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/728653004

Cr-Commit-Position: refs/heads/master@{#25418}