clang/win: use clang_warning_flags, like mac and linux do

clang/win: use clang_warning_flags, like mac and linux do

In addition to disabling warnings, this does turn on a few warnings that weren't
on before (-Wheader-hygiene, -Wstring-conversions). I'll see how our bot copes,
if it's too horrible I might revert this again for a bit.

BUG=82385
NOTRY=true

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=287237

[Nico, 2014-08-01 21:59:10]

I don't have a windows box at hand to test this today, and I don't think we have
trybots.

This adds -Wheader-hygiene and -Wstring-conversions, so it's possible it's going
to add warnings (and not everything uses the clang_warning_flags thingy yet)

[hans, 2014-08-01 23:28:33]

lgtm

[Nico, 2014-08-03 00:08:42]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2014-08-03 00:08:57]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/thakis@chromium.org/419963004/1

[Nico, 2014-08-03 01:04:39]

The CQ bit was unchecked by thakis@chromium.org

[Nico, 2014-08-03 01:04:47]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2014-08-03 01:04:50]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/thakis@chromium.org/419963004/1

[commit-bot: I haz the power, 2014-08-03 01:05:38]

Change committed as 287237