The generate-header-include-checks.py script generates the cc files to
check that headers can be included in isolation. It currently always
writes to all the cc files. This means that after "gclient runhooks",
all these files have to be recompiled.
This CL fixes this by not touching the file if it already contains the
expected content. This avoids hundreds of recompilations after running
the hook.

R=machenbach@chromium.org

Bug: v8:7965
Change-Id: I93a0abb936863f6c21ed22ba1b7d900b96725fa3
Reviewed-on: https://chromium-review.googlesource.com/1156514
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54858}