Add tools for code coverage support in iOS.

build/config/ios/BUILD.gn

Index: build/config/ios/BUILD.gn
diff --git a/build/config/ios/BUILD.gn b/build/config/ios/BUILD.gn
index ec6b9e67bcfb5f1fa5a7acb1495f5d4e6d7d8ac7..8dca86227bac485cfac9ce7bb749613e055b2c07 100644
--- a/build/config/ios/BUILD.gn
+++ b/build/config/ios/BUILD.gn
@@ -65,6 +65,10 @@ config("runtime_library") {
     cflags_cc = common_cc_flags
     cflags_objcc = common_cc_flags
   }
+
+  if (ios_enable_coverage) {
+    configs = [ ":enable_coverage" ]
+  }
 }
 
 config("ios_dynamic_flags") {
@@ -86,6 +90,16 @@ config("xctest_config") {
   ]
 }
 
+# This enables support for LLVM code coverage. See
+# http://llvm.org/docs/CoverageMappingFormat.html.
+config("enable_coverage") {
+  cflags = [
+    "-fprofile-instr-generate",
+    "-fcoverage-mapping",
+  ]
+  ldflags = [ "-fprofile-instr-generate" ]
+}
+
 group("xctest") {
   public_configs = [ ":xctest_config" ]
 }