ninja windows: Make pch work on VS2012, and simplify implementation

Previously, the chromium .gypi files injected a .cc file for PCH, which compiled "normally", and then the generator compiled it again (to a different .obj). Now, the build rule for the injected .cc has its ccflags modified to cause generation of the pch, which matches the msvs generator. Also get to remove the ugly separate 'cc_pch'/'cxx_pch' rules.

I believe this will also fix some very unreproducible build errors we were having (the linked bug) because the dependencies for the pch pdb is much simpler.

BUG=142362, 143646
Review URL: https://codereview.chromium.org/12052029

git-svn-id: http://gyp.googlecode.com/svn/trunk@1565 78cadc50-ecff-11dd-a971-7dbc132099af