Add -j as alias for --threads.

I keep typing this...

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4553

Change-Id: Ib3e387054810cebae468addc001a1e148e5af155
Reviewed-on: https://skia-review.googlesource.com/4553
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/tools/flags/SkCommonFlags.cpp b/tools/flags/SkCommonFlags.cpp
index d0cdf31..a0aa94b 100644
--- a/tools/flags/SkCommonFlags.cpp
+++ b/tools/flags/SkCommonFlags.cpp
@@ -50,8 +50,8 @@
 
 DEFINE_string(svgs, "", "Directory to read SVGs from, or a single SVG file.");
 
-DEFINE_int32(threads, -1, "Run threadsafe tests on a threadpool with this many extra threads, "
-                          "defaulting to one extra thread per core.");
+DEFINE_int32_2(threads, j, -1, "Run threadsafe tests on a threadpool with this many extra threads, "
+                               "defaulting to one extra thread per core.");
 
 DEFINE_bool2(verbose, v, false, "enable verbose output from the test driver.");