Use PCH by default in VS 2010.

Use PCH by default in VS 2010.

According to my testing, VS 2010 does not have problem (b) documented
at http://code.google.com/p/chromium/wiki/WindowsPrecompiledHeaders,
i.e. it rebuilds the PCH file when necessary because of
e.g. preprocessor flag changes.

Since we aren't using VS 2010 for official builds, if problem (a) (out
of memory) is encountered with VS 2010 when we make the switch to
using it for official builds, we can simply turn PCH off explicitly in
the include.gypi for the official bots.

This change has no effect on VS 2008 bots or users.

BUG=110001
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=117845

[M-A Ruel, 2012-01-12 14:34:13]

lgtm, thanks.

[commit-bot: I haz the power, 2012-01-16 10:30:52]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/joi@chromium.org/9139059/7001

[commit-bot: I haz the power, 2012-01-16 11:45:20]

Change committed as 117845