Introduce skia_pic gyp variable.

gyp/common_variables.gypi

Index: gyp/common_variables.gypi
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index c3714dc6c038757d152daaa085d700464c281397..24f0e879fa50bf90a75764846e8eef85a3c8eb90 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -180,6 +180,13 @@
         'skia_clang_build%': 0,
         'skia_keep_frame_pointer%': 0,
       }],
+      [ 'skia_shared_lib == 1 or skia_sanitizer == "thread" '
+        'or skia_sanitizer == "undefined" or skia_os == "android"', {

[djsollen, 2014/07/18 19:02:25]

does the sanitizer stuff need to be guarded by...
'skia_os in ["linux", "freebsd", "openbsd", "solaris", "nacl", "chromeos"

[scroggo, 2014/07/18 19:06:43]

Good question. I'm guessing we don't ever build the sanitizer unless skia_os is
one of those.

If we add all those checks, might it be cleaner to leave them alone (e.g.
continue to set -fPIC for them inside their own conditions blocks and ignore
them here)?

[mtklein, 2014/07/19 14:49:19]

Right.  No need to guard.  It won't ever be set on OS/toolchains that don't
support it.  For what it's worth, some xSANs work on Linux and Mac now, and will
work on Windows once we get clang there.
If it makes things simpler to think about, I think it's safe to simplify this
condition to 'skia_shared_lib or skia_sanitizer or skia_os == "android"'.  That
is, I think position-independent code should be fine for any sanitizer mode;
it's just _required_ for TSAN and UBSAN.  Right now we only additionally use
ASAN, and I don't think it'll mind being PIC.

As far as testing on our bots goes, I'd be willing to go all the way to making
-fPIC the default in all Skia gyps.  The only place we really want to turn it
off is for things like linking into Chrome, to save space and run a smidge
faster.

[scroggo, 2014/07/22 14:53:36]

Done.
It seems like there are two ways to do this:
- default to skia_pic, but turn it off in chrome (or does chrome ignore
  our common variables? then it just wouldn't be set)
- add it for all of our targets

+          'skia_pic%' : 1,
+        }, {
+          'skia_pic%' : 0,
+        }
+      ],
     ],
 
     # Re-define all variables defined within the level-2 'variables' dict,