Revert of win: Set is_clang=true by default. (patchset #6 id:100001 of https://codereview.chromium.org/2870543003/ )

Reason for revert:
Breaks many tests:
goma errors https://bugs.chromium.org/p/chromium/issues/detail?id=727488
webkit_tests is https://bugs.chromium.org/p/chromium/issues/detail?id=727463
(most of) win7 tests is https://bugs.chromium.org/p/chromium/issues/detail?id=727458

Original issue's description:
> win: Set is_clang=true by default.
>
> To figure out unknown problems. If this inconveniences you in any way, please
> do let us know (ideally on crbug.com) – finding annoying things are part of the
> motivation for this change.
>
> If this breaks anything, please revert with prejudice -- but file a bug for
> us about what went wrong. We intend to revert this in 5 hours if it's not
> reverted earlier due to breaking something.
>
> This also switches win_clang to use MSVC as compiler, to make sure things stay
> compilable with both compilers even while this is in.
>
> Known problems:
> * build times without goma are slower (but you can do goma builds with debug
>   info, which are faster)
> * incremental linking of webcore.dll doesn't work, so if you work on blink
>   on windows links will be slow. If this affects you, please star
>   https://crbug.com/717207 and make yourself heard.
>
> This doesn't affect what we ship, which is currently off an PGO builder.
>
> BUG=709690,82385
>
> Review-Url: https://codereview.chromium.org/2870543003
> Cr-Commit-Position: refs/heads/master@{#475401}
> Committed: https://chromium.googlesource.com/chromium/src/+/d560ed865c053d87594aca8a72637593117d5d1b

TBR=hans@chromium.org,brettw@chromium.org,brucedawson@chromium.org,jam@chromium.org,thakis@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=709690,82385

Review-Url: https://codereview.chromium.org/2907363002
Cr-Commit-Position: refs/heads/master@{#475447}
2 files changed