Revert 153092 - Turn on warnings as errors for (most) third_party code on Windows

Revert 153092 - Turn on warnings as errors for (most) third_party code on Windows

Is causing warnings for Official build in PDF code. Boo.

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Can't quite be fully enabled yet due to not-yet-fixed third party
dependencies. Without it enabled, other packages regress while we're fixing
things. So, add a flag for now so warnings-freeness can be ratcheted forward
by having it on for most packages, but off for a few.

Also, disable warning in qcms (fixed upstream by a large refactoring, not
worth rolling for), and disable two minor warnings in yasm (patch posted
upstream for a few months, but maintainer does not seem motivated to merge).

Fix release-only warning in leveldatabase/env_chromium.cc. Was calling exit(1)
in a leaky destructor.

Fix a warning in lzma_sdk (missing an include).

Disable a silly warning in Release builds of skia and memory_watcher (that
/GS is not working because optimization is disabled).

Warning are currently tolerated in libvpx and mesa.

Cannot be committed until http://chromiumcodereview.appspot.com/10823426/ has
landed.

R=thakis@chromium.org
BUG=126483, 140121, 143877
TEST=


Review URL: https://chromiumcodereview.appspot.com/10827429

TBR=scottmg@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=153223