Fix ThinLTO for CrOS

This fixes a bug in I8a928eaab93ec181ba057cdda4cb635389f30926, which
breaks ThinLTO builds for CrOS with an unused variable.

Bug: 807147

TBR=agreive@chromium.org, rnk@chromium.org

Change-Id: I19b7cf81f035009d5390e9fc3f12f6e29413aa24
Reviewed-on: https://chromium-review.googlesource.com/1190904Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586336}

build/config/compiler/BUILD.gn

@@ -682,6 +682,8 @@ config("compiler") {
             ]
           }
         }
+      } else {
+        not_needed([ "lto_opt_level" ])
       }
     }