Move write barrier essentials into heap/heap-write-barrier-inl.h. Avoid
including further heap inline headers by relying on constant to load
flags from.

Bug: v8:7490
Change-Id: I2891299f1b1ca2c3e2031cb9c63b583b1665e3f9
Reviewed-on: https://chromium-review.googlesource.com/1148448
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54710}