Adding a tool to get images from skps

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1696763002

Review URL: https://codereview.chromium.org/1696763002
diff --git a/tools/flags/SkCommandLineFlags.cpp b/tools/flags/SkCommandLineFlags.cpp
index cddf9fb..3dd746c 100644
--- a/tools/flags/SkCommandLineFlags.cpp
+++ b/tools/flags/SkCommandLineFlags.cpp
@@ -150,6 +150,10 @@
     gUsage.set(usage);
 }
 
+void SkCommandLineFlags::PrintUsage() {
+    SkDebugf("%s", gUsage.c_str());
+}
+
 // Maximum line length for the help message.
 #define LINE_LENGTH 72