Factor out skia_keep_frame_pointer.  Handy with perf to get call stacks.

Today it's set when we're running a sanitizer, but it's more generally useful.  perf record -g (--call-graph) only works for me if we don't omit the frame pointer.

BUG=
R=bungeman@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/41923004

git-svn-id: http://skia.googlecode.com/svn/trunk@11966 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index 9aeee62..579ce70 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -120,8 +120,10 @@
       }],
       [ 'skia_sanitizer', {
         'skia_clang_build': 1,
+        'skia_keep_frame_pointer': 1,
       }, {
         'skia_clang_build%': 0,
+        'skia_keep_frame_pointer%': 0,
       }],
     ],