win: Pass -ftime-trace without /clang: prefix.

It's been a CoreOption since https://reviews.llvm.org/D61945
No behavior change.

Bug: none
Change-Id: I32acae26fffb3b71fd9d866aead3569c7ace7287
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547204
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828784}

build/config/compiler/BUILD.gn

@@ -694,11 +694,7 @@ config("compiler") {
 
   if (compiler_timing) {
     if (is_clang && !is_nacl) {
-      if (is_win) {
-        cflags += [ "/clang:-ftime-trace" ]
-      } else {
-        cflags += [ "-ftime-trace" ]
-      }
+      cflags += [ "-ftime-trace" ]
     } else if (is_win) {
       cflags += [
         # "Documented" here: