Enable -Wshift-negative-value.

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 83d649b4c44ad5724d446fae9df91943c2e4422c..7cc1ae43f69c5ad43717ab7360a496cf020cedad 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1078,17 +1078,6 @@ config("default_warnings") {
       "-Wno-inconsistent-missing-override",
     ]
 
-    # Chrome's hermetic Clang compiler, NaCl's Clang compiler and Xcode's Clang
-    # compiler will almost always have different versions. Certain flags may not
-    # be recognized by one version or the other.
-    if (!is_nacl) {
-      # Flags NaCl (Clang 3.7) does not recognize.
-      cflags += [
-        # TODO(thakis): Enable this, crbug.com/507717
-        "-Wno-shift-negative-value",
-      ]
-    }
-
     # use_xcode_clang only refers to the iOS toolchain, host binaries use
     # chromium's clang always.
     if (!is_nacl && (!use_xcode_clang || current_toolchain == host_toolchain)) {