Linux: add option to use system nspr.

base/base.gyp

Index: base/base.gyp
diff --git a/base/base.gyp b/base/base.gyp
index a584ae425b1308ef5c12acb56c85fae979354846..8c354e083b34293c6829d8285a54dc882d517b3a 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -206,6 +206,11 @@
             }],
           ],
         }],
+        ['use_system_nspr==1', {
+          'dependencies': [
+            'third_party/nspr/nspr.gyp:nspr',
+          ],
+        }],
       ],
       'sources': [
         'third_party/nspr/prcpucfg.h',
@@ -716,6 +721,11 @@
             'win/win_util_unittest.cc',
           ],
         }],
+        ['use_system_nspr==1', {
+          'dependencies': [
+            'third_party/nspr/nspr.gyp:nspr',
+          ],
+        }],
       ],  # conditions
       'target_conditions': [
         ['OS == "ios"', {