Make clang_load and clang_add_plugin independent.
Make clang_load and clang_add_plugin independent.

BUG=none
TEST=GYP_DEFINES=clang_add_plugin=foo build/gyp_chromium adds a parameter


Review URL: http://codereview.chromium.org/8772034

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@112630 0039d316-1c4b-4281-b951-d872f2087c98

adobe/chromium,yitian134/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,ropik/chromium,adobe/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,ropik/chromium