Adding new benchmark to test image decoding performance.

BUG=skia:

Review URL: https://codereview.chromium.org/918673002
diff --git a/tools/flags/SkCommonFlags.cpp b/tools/flags/SkCommonFlags.cpp
index 2a0490c..15d9123 100644
--- a/tools/flags/SkCommonFlags.cpp
+++ b/tools/flags/SkCommonFlags.cpp
@@ -23,6 +23,8 @@
               "Defaults to empty string, which selects the API native to the "
               "system.");
 
+DEFINE_string(images, "resources", "Directory of images to decode.");
+
 DEFINE_bool2(leaks, l, false, "show leaked ref cnt'd objects.");
 
 DEFINE_string2(match, m, NULL,