Introduce skia_pic gyp variable.

skia_pic tells ninja to use -fPIC when building for position
independent code.

Set skia_pic to true when building our existing targets that
require position independent code.

Also use skia_pic when building for Android.

R=mtklein@google.com, djsollen@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/406613003
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index e084cd9..b7c82e3 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -181,6 +181,12 @@
         'skia_clang_build%': 0,
         'skia_keep_frame_pointer%': 0,
       }],
+      [ 'skia_shared_lib or skia_sanitizer or skia_os == "android"', {
+          'skia_pic%' : 1,
+        }, {
+          'skia_pic%' : 0,
+        }
+      ],
     ],
 
     # Re-define all variables defined within the level-2 'variables' dict,