Update V8 DEPS.

Rolling v8/tools/clang to 6538d768c1dd43ad3942574cfc5ba90a8e1e0517

TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/803183005

Cr-Commit-Position: refs/heads/master@{#25830}
Revert of [turbofan] Relax effects and context for JSToNumber(x:plain-primitive). (patchset #1 id:1 of https://codereview.chromium.org/799413002/)

Reason for revert:
Performance regressions on primes and corrections benchmarks.

Original issue's description:
> [turbofan] Relax effects and context for JSToNumber(x:plain-primitive).
>
> TEST=unittests
> R=svenpanne@chromium.org
>
> Committed: https://chromium.googlesource.com/v8/v8/+/75484e8d16866eba7aa9c3b87841cd6ce2f466b8

TBR=svenpanne@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/806103002

Cr-Commit-Position: refs/heads/master@{#25831}
[turbofan] Relax effects and context for JSToNumber(x:plain-primitive).

Relanded with fix for always returning Change for PlainPrimitive even
if there was no change. The performance regression on primes.js and
corrections.js is due to unlucky loop header alignment; will be addressed
separately.

TEST=unittests
R=svenpanne@chromium.org

Committed: https://chromium.googlesource.com/v8/v8/+/75484e8d16866eba7aa9c3b87841cd6ce2f466b8

Review URL: https://codereview.chromium.org/799413002

Cr-Commit-Position: refs/heads/master@{#25832}