Made OSMesa shared library position independent on linux.

This is an attempt to fix this valgrind error:
http://build.chromium.org/p/chromium.memory/builders/Linux%20Tests%20(valgrind)(2)/builds/210/steps/memory%20test:%20ui_1/logs/stdio

It appears mmap might have been trying to map libosmesa.so to a fixed address.

TEST=try
BUG=61601

Review URL: http://codereview.chromium.org/4318005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@64941 0039d316-1c4b-4281-b951-d872f2087c98

third_party/mesa/mesa.gyp

@@ -19,6 +19,11 @@
           'HAVE_STRNLEN',
         ],
       }],
+      ['OS=="linux"', {
+        'cflags': [
+          '-fPIC',
+        ],
+      }],
     ],
     'defines': [
       # For Mesa

tools/valgrind/gtest_exclude/ui_tests.gtest.txt

@@ -53,7 +53,3 @@ NewTabUITest.NTPHasLoginName
 # Mac: See http://crbug.com/30661, Linux/CrOS: http://crbug.com/49576.
 LayoutPluginTester.NPObjectReleasedOnDestruction
 LayoutPluginTester.NPObjectSetException
-
-# Fails on valgrind
-# http://crbug.com/61601
-GPUUITest.UITestLaunchedWithOSMesa