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

Review URL: https://codereview.chromium.org/314163003

git-svn-id: e7e1075985beda50ea81ac4472467b4f6e91fc78@1929 78cadc50-ecff-11dd-a971-7dbc132099af

svn2github/gyp,svn2github/kgyp,svn2github/gyp,svn2github/gyp,pyokagan/gyp,luvit/gyp,svn2github/gyp,luvit/gyp,pyokagan/gyp,svn2github/kgyp,luvit/gyp,svn2github/kgyp,svn2github/kgyp,luvit/gyp,pyokagan/gyp,svn2github/kgyp,pyokagan/gyp,svn2github/gyp,pyokagan/gyp