Fixed all unused-variable Clang warnings on Windows.

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 7574a137a3f225b30753159e9d8cc97f1c9c9b05..8914bf1443eb3c7bd3f3f0c1167764a14731b948 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -817,7 +817,6 @@ if (is_win) {
       "-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
     ]
   }
@@ -998,6 +997,12 @@ config("no_chromium_code") {
     cflags += [ "-Wno-unused-result" ]
   }
 
+  if (is_clang) {
+    # Lots of third-party libraries have unused variables. Instead of
+    # suppressing them individually, we just blanket suppress them here.
+    cflags += [ "-Wno-unused-variable" ]
+  }
+
   if (is_linux || is_android) {
     cflags += [
       # Don't warn about printf format problems. This is off by default in gcc