Re-enable -Wstring-concatenation

Bug: 1114873
Change-Id: I45701a5d484481a54514b367ece99259abc64a5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366552
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804226}

build/config/compiler/BUILD.gn

@@ -1541,9 +1541,6 @@ config("default_warnings") {
 
           # TODO(https://crbug.com/1050281): Clean up, enable.
           "-Wno-non-c-typedef-for-linkage",
-
-          # TODO(https://crbug.com/1114873): Clean up, enable.
-          "-Wno-string-concatenation",
         ]
 
         cflags_c += [