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.cpp b/tools/flags/SkCommonFlags.cpp
index 80ef2bf..4d08ba6 100644
--- a/tools/flags/SkCommonFlags.cpp
+++ b/tools/flags/SkCommonFlags.cpp
@@ -52,3 +52,5 @@
 DEFINE_bool2(verbose, v, false, "enable verbose output from the test driver.");
 
 DEFINE_bool2(veryVerbose, V, false, "tell individual tests to be verbose.");
+
+DEFINE_string2(writePath, w, "", "If set, write bitmaps here as .pngs.");
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