Define GR_GL_USE_NEW_SHADER_SOURCE_SIGNATURE to fix servo/servo#3944.

ToT of servo/skia currently breaks with the below errors when building the android port of servo:
<blockquote>
src/gpu/gl/android/GrGLCreateNativeInterface_android.cpp: In function 'const GrGLInterface* GrGLCreateNativeInterface()':
src/gpu/gl/android/GrGLCreateNativeInterface_android.cpp:79:36: error: invalid conversion from 'void (*)(GLuint, GLsizei, const GLchar* const*, const GLint*) {aka void (*)(unsigned int, int, char const* const*, int const*)}' to 'void (*)(unsigned int, int, char const**, int const*)' [-fpermissive]
include/gpu/gl/GrGLInterface.h:116:15: error:   initializing argument 1 of 'GrGLInterface::GLPtr<FNPTR_TYPE> GrGLInterface::GLPtr<FNPTR_TYPE>::operator=(FNPTR_TYPE) [with FNPTR_TYPE = void (*)(unsigned int, int, char const**, int const*), GrGLInterface::GLPtr<FNPTR_TYPE> = GrGLInterface::GLPtr<void (*)(unsigned int, int, char const**, int const*)>]' [-fpermissive]
make[1]: *** [/Volumes/lab101/FOSS/Servo/ports/android/glut_app/target/arm-linux-androideabi/native/skia-sys-60a65b28f3cfaa09/src/gpu/gl/android/GrGLCreateNativeInterface_android.o] Error 1
make[1]: *** Waiting for unfinished jobs....
</blockquote>
It seems that servo/skia needs to have GR_GL_USE_NEW_SHADER_SOURCE_SIGNATURE on android.

My development environment is:
<blockquote>
Mac OS X Yosemite 10.10<br>
Ubuntu 14.04 64 Bits<br>
Android SDK 21<br>
Android NDK r10c<br>
GCC 4.8.2
</blockquote>