Fix msvs-ninja clean build command / target.

Fix msvs-ninja clean build command / target.

Currently the clean command is specified as

ninja.exe -C $(OutDir) -t clean $(ProjectName)

but _BuildCommandLineForRuleRaw sees 'clean' as a path, calling _FixPath on it.
This is not a problem for Chromium because it does not set --generator-output.
As a result fixpath_prefix is None and no adjustment is made to paths.
Skia, however, does set --generator-output, and so an attempt is made to adjust
paths with _FixPath. As a result, with Skia the msvs 'Clean' target contains

ninja.exe -C $(OutDir) -t ../../gyp/clean $(ProjectName)

This change changes the command to

ninja.exe -C $(OutDir) -tclean $(ProjectName)

which it valid for ninja, but _BuildCommandLineForRuleRaw will not _FixPath it.

BUG=skia:2439
R=sbaig1@bloomberg.net

Committed: https://code.google.com/p/gyp/source/detail?r=1929

[Shezan Baig (Bloomberg), 2014-06-05 17:30:47]

lgtm, thanks!

[bungeman-chromium, 2014-06-05 18:26:36]

Committed patchset #1 manually as r1929.