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

gyp/gpu.gyp

Index: gyp/gpu.gyp
diff --git a/gyp/gpu.gyp b/gyp/gpu.gyp
index e835e34850a77c320ea4709b596296f2060ac50c..c69a4867d662942fb20258ca892ebc9a8342e804 100644
--- a/gyp/gpu.gyp
+++ b/gyp/gpu.gyp
@@ -108,6 +108,11 @@
         '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',