Add ability to pass in an optional path to additional unit tests to run.

BUG=skia:

Review URL: https://codereview.chromium.org/1280133004
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index 21fb09d..ed4cdb6 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -123,6 +123,7 @@
       'skia_scalar%': 'float',
       'skia_mesa%': 0,
       'skia_gpu_extra_dependency_path%': '',
+      'skia_gpu_extra_tests_path%': '',
       'skia_stroke_path_rendering%': 0,
       'skia_android_path_rendering%': 0,
       'skia_resource_cache_mb_limit%': 0,
@@ -193,6 +194,7 @@
     'skia_scalar%': '<(skia_scalar)',
     'skia_mesa%': '<(skia_mesa)',
     'skia_gpu_extra_dependency_path%': '<(skia_gpu_extra_dependency_path)',
+    'skia_gpu_extra_tests_path%': '<(skia_gpu_extra_tests_path)',
     'skia_stroke_path_rendering%': '<(skia_stroke_path_rendering)',
     'skia_android_framework%': '<(skia_android_framework)',
     'skia_use_android_framework_defines%': '<(skia_use_android_framework_defines)',