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

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index f589cffe5c02d1863a1614438d4c1166b92cf980..4e7df4aef786c9acfa0d3a1169aa08deb9abfb8b 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -808,6 +808,7 @@
       "-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
     ]
   }
@@ -1004,10 +1005,6 @@
       # 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",
     ]
   }