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

BUG=skia:

Review URL: https://codereview.chromium.org/986363002

gyp/common_variables.gypi

@@ -138,6 +138,7 @@
       'skia_sanitizer%': '',
       'skia_scalar%': 'float',
       'skia_mesa%': 0,
+      'skia_gpu_extra_dependency_path%': '',
       'skia_stroke_path_rendering%': 0,
       'skia_android_path_rendering%': 0,
       'skia_resource_cache_mb_limit%': 0,
@@ -215,6 +216,7 @@
     'skia_sanitizer%': '<(skia_sanitizer)',
     'skia_scalar%': '<(skia_scalar)',
     'skia_mesa%': '<(skia_mesa)',
+    'skia_gpu_extra_dependency_path%': '<(skia_gpu_extra_dependency_path)',
     'skia_stroke_path_rendering%': '<(skia_stroke_path_rendering)',
     'skia_android_framework%': '<(skia_android_framework)',
     'skia_use_system_json%': '<(skia_use_system_json)',

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',