Allow user to pass in a path for extra dependencies for gpu

gyp/gpu.gyp

 # GYP for building gpu
 {
   'target_defaults': {
     'conditions': [
       ['skia_os != "win"', {
         'sources/': [ ['exclude', '_win.(h|cpp)$'],
         ],
       }],
       ['skia_os != "mac"', {
         'sources/': [ ['exclude', '_mac.(h|cpp|m|mm)$'],
@@ skipping 90 matching lines @@
       'sources': [
         '<@(skgpu_sources)',
         '<@(skgpu_native_gl_sources)',
         '<@(skgpu_angle_gl_sources)',
         '<@(skgpu_mesa_gl_sources)',
         '<@(skgpu_debug_gl_sources)',
         '<@(skgpu_null_gl_sources)',
         'gpu.gypi', # Makes the gypi appear in IDEs (but does not modify the build).
       ],
       'conditions': [
+        [ 'skia_gpu_extra_dependency_path', {
+          'dependencies' : [
+              '<(skia_gpu_extra_dependency_path):*',
+          ]
+        }],
         [ 'skia_stroke_path_rendering', {
           'sources': [
             '../experimental/StrokePathRenderer/GrStrokePathRenderer.h',
             '../experimental/StrokePathRenderer/GrStrokePathRenderer.cpp',
           ],
           'defines': [
             'GR_STROKE_PATH_RENDERING=1',
           ],
         }],
         [ 'skia_android_path_rendering', {
@@ skipping 121 matching lines @@
             'libraries': [
               '-lGLESv2',
               '-lEGL',
             ],
           },
         }],
       ],
     },
   ],
 }