Revert of Clang on Windows: Remove -Wno-unused-variable.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index c4b652da23cbfb3080918887a90f4ff37c4c21cf..6a13f95a4662322e8acfe4acc3baf768b7c4bb8b 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -3111,9 +3111,6 @@
             # TODO(mgiuca): Move this suppression into individual third-party
             # libraries as required. http://crbug.com/505301.
             '-Wno-overloaded-virtual',
-            # Lots of third-party libraries have unused variables. Instead of
-            # suppressing them individually, we just blanket suppress them here.
-            '-Wno-unused-variable',
           ],
         },
         'conditions': [
@@ -5738,6 +5735,7 @@
                   '-Wno-unused-function',  # http://crbug.com/505316
                   '-Wno-unused-private-field',  # http://crbug.com/505317
                   '-Wno-unused-value',  # http://crbug.com/505318
+                  '-Wno-unused-variable',  # http://crbug.com/505319
                   '-Wno-unused-local-typedef',  # http://crbug.com/411648
                 ],
               },