Enable -Wundeclared-selector on ios.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index f3a69627b3b04726b55af755978481a6a7cd2ff7..28214c87ca4ddfed2a4716e2a98d20e20eb9627c 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -970,6 +970,12 @@ config("default_warnings") {
       cflags += [ "-Wpartial-availability" ]
     }
 
+    if (is_ios) {

[stkhapugin, 2016/12/23 10:27:18]

Can you ping someone in Chromium on Mac to look at this in case they want this
flag?

[marq (ping after 24h), 2016/12/23 10:33:06]

I will, but if the Mac code requires fixes as well, I don't want to complicate
this CL with them. So my thinking is that we can land this and start using it
for iOS, and if the Mac folks want to turn it on they can.

+      # When compiling Objective-C, warns if a selector named via @selector has
+      # not been defined in any visible interface.
+      cflags += [ "-Wundeclared-selector" ]
+    }
+
     # Suppress warnings about ABI changes on ARM (Clang doesn't give this
     # warning).
     if (current_cpu == "arm" && !is_clang) {