win: Pass -fno-delete-null-pointer-checks without /clang: prefix.

As of the clang roll in https://crbug.com/1149189, clang-cl understands
this flag directly.

Bug: 1139129
Change-Id: I5f484d259f48855967a1715e87d2f704826ceacd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547122
Commit-Queue: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828914}

build/config/compiler/BUILD.gn

@@ -266,12 +266,7 @@ config("compiler") {
   # Nacl does not support the flag. And, we still want UBSAN to catch undefined
   # behavior related to nullptrs, so do not add this flag if UBSAN is enabled.
   if (!is_nacl && !is_ubsan) {
-    if (is_win) {
-      # TODO(thakis): Remove this branch once chromium's clang has https://github.com/llvm/llvm-project/commit/3109ce5
-      cflags += [ "/clang:-fno-delete-null-pointer-checks" ]
-    } else {
-      cflags += [ "-fno-delete-null-pointer-checks" ]
-    }
+    cflags += [ "-fno-delete-null-pointer-checks" ]
   }
 
   # In general, Windows is totally different, but all the other builds share