Fix iOS build by centralizing --writePath.

bench/nanobench.cpp

Index: bench/nanobench.cpp
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index 245aa0ed1dda90d9bb845fca28aee230e0b24532..a5353aa6e845bf0717fe4432eb4340d0d5a246bd 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -33,7 +33,7 @@ __SK_FORCE_IMAGE_DECODER_LINKING;
 
 static const int kAutoTuneLoops = -1;
 
-static const int kDefaultLoops = 
+static const int kDefaultLoops =
 #ifdef SK_DEBUG
     1;
 #else
@@ -50,8 +50,6 @@ static SkString loops_help_txt() {
 
 DEFINE_int32(loops, kDefaultLoops, loops_help_txt().c_str());
 
-DEFINE_string2(writePath, w, "", "If set, write benches here as .pngs.");
-
 DEFINE_int32(samples, 10, "Number of samples to measure for each bench.");
 DEFINE_int32(overheadLoops, 100000, "Loops to estimate timer overhead.");
 DEFINE_double(overheadGoal, 0.0001,