Create a define for nanobench builds to dump stats

TBR=bsalomon@google.com
BUG=skia:

Review URL: https://codereview.chromium.org/1499453002
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index 64bc29f..56ae72e 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -238,6 +238,7 @@
     'skia_egl%': '<(skia_egl)',
     'skia_use_sdl%': 0,
     'skia_fast%': 0,
+    'skia_dump_stats%': 0,
     'skia_fast_flags': [
         '-O3',                   # Even for Debug builds.
         '-march=native',         # Use all features of and optimize for THIS machine.