Remove use_system_nspr build flag. It's not "officially" supported

base/base.gyp

Index: base/base.gyp
diff --git a/base/base.gyp b/base/base.gyp
index 78e6ddb0227291275a47d6c275fc4ff9704272ad..7e12b9537bf5143cddb4e110818295913d6d7cf9 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -234,11 +234,6 @@
             }],
           ],
         }],
-        ['use_system_nspr==1', {
-          'dependencies': [
-            'third_party/nspr/nspr.gyp:nspr',
-          ],
-        }],
       ],
       'sources': [
         'third_party/nspr/prcpucfg.h',
@@ -811,11 +806,6 @@
             'x11/edid_parser_x11_unittest.cc',
           ],
         }],
-        ['use_system_nspr==1', {
-          'dependencies': [
-            'third_party/nspr/nspr.gyp:nspr',
-          ],
-        }],
       ],  # conditions
       'target_conditions': [
         ['OS == "ios" and _toolset != "host"', {