This reverts commit 25207bf8.

Reason for revert: Mips compile failure: https://build.chromium.org/p/client.v8.ports/builders/V8%20Mips%20-%20builder/builds/16061

Original change's description:
> Remove isolate.h include from external-reference.h
> 
> This triggers a bunch of other necessary include tweaks for files that
> used to work until now because they indirectly included isolate.h
> through external-reference.h.
> 
> Bug: v8:6666
> Change-Id: I8d48db44dcc321fa32a6279f3ddacb41ab58f975
> Reviewed-on: https://chromium-review.googlesource.com/972042
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Peter Marshall <petermarshall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52103}

TBR=jgruber@chromium.org,petermarshall@chromium.org

Change-Id: I2e867d3c82b020b4fd5201aaa5cf6ed65b081ca2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/973061Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52104}