Roll clang 289944:295762.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 5722c125d0073196a0476e37541ecf9f9493aa9e..6eb3c9080393b1b80c79650257bcdd0b5dd745d9 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -50,12 +50,7 @@ hermetic_xcode_path =
 declare_args() {
   if (is_clang) {
     # Clang compiler version. Clang files are placed at version-dependent paths.
-    if (llvm_force_head_revision) {
-      clang_version = "5.0.0"
-    } else {
-      # TODO(thakis): Remove this branch the next time clang rolls.
-      clang_version = "4.0.0"
-    }
+    clang_version = "5.0.0"
   }
 }