When libyuv was changed to compile with clang-cl this also cause libjpeg
to be compiled with clang-cl, which means that it was being compiled
twice. This is generally a bad practice, it was causing duplicate symbol
warnings in some builds, and it slows builds slightly. This change
reduces the number of build steps with the default gn settings on
Win64 builds by 71 (currently from 47,200 to 47,129).

See this change that change libyuv's compilation:
https://chromium-review.googlesource.com/c/446667/

BUG=706627

Change-Id: I13e2d4ff8511350901af10f7443f3f4b89cc2499
Reviewed-on: https://chromium-review.googlesource.com/456651Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>