Clang Win: turn on -Wno-ignored-pragmas

BUG=644841
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/2316343002 .

Cr-Commit-Position: refs/heads/master@{#417101}

build/config/compiler/BUILD.gn

@@ -937,6 +937,7 @@ config("default_warnings") {
         "-Wno-microsoft-enum-value",  # http://crbug.com/505296
         "-Wno-unknown-pragmas",  # http://crbug.com/505314
         "-Wno-microsoft-cast",  # http://crbug.com/550065
+        "-Wno-ignored-pragmas",  # http://crbug.com/644841
       ]
     }
   } else {