Fix iOS build by centralizing --writePath.

CQ_EXTRA_TRYBOTS=tryserver.skia:Build-Mac10.7-Clang-Arm7-Debug-iOS-Trybot

BUG=skia:
R=mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/452633002
diff --git a/tools/flags/SkCommonFlags.h b/tools/flags/SkCommonFlags.h
index 4abf201..b45ef07 100644
--- a/tools/flags/SkCommonFlags.h
+++ b/tools/flags/SkCommonFlags.h
@@ -26,5 +26,6 @@
 DECLARE_string(resourcePath);
 DECLARE_bool(verbose);
 DECLARE_bool(veryVerbose);
+DECLARE_string(writePath);
 
 #endif