Bug 1076743 - Fixed angle compilation with mingw. (upstream part)

--HG--
rename : gfx/angle/src/libGLESv2/constants.h => gfx/angle/src/libGLESv2/Constants.h
extra : rebase_source : 3147126bc9ccd9df61f4f27e91efe397a6538943
Bug 1076743 - Fixed angle compilation with mingw. (moz.build part) r=gps

--HG--
extra : rebase_source : 5bff5b899c83796443c124dce0f5afbd16e611c2