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 71f87de3ff914a0112b14dcf86933f1842b5fcf1..f8091d155b05006e774caf5773ed82c091c401c0 100644
--- a/build/config/ios/BUILD.gn
+++ b/build/config/ios/BUILD.gn
@@ -112,6 +112,10 @@ config("runtime_library") {
     cflags_cc = common_cc_flags
     cflags_objcc = common_cc_flags
   }
+
+  if (ios_enable_coverage) {
+    configs = [ ":enable_coverage" ]
+  }
 }
 
 config("ios_executable_flags") {
@@ -136,6 +140,16 @@ 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" ]
 }