Add tools for code coverage support in iOS.

ios/showcase/BUILD.gn

Index: ios/showcase/BUILD.gn
diff --git a/ios/showcase/BUILD.gn b/ios/showcase/BUILD.gn
index 7dda133ab4f394b412a1f18a7e446e1bd4d2dd8f..b1b83cdc2bc914d162df346ad68cb779cbbe9df3 100644
--- a/ios/showcase/BUILD.gn
+++ b/ios/showcase/BUILD.gn
@@ -19,6 +19,7 @@ ios_app_bundle("showcase") {
     # asan mode (this target will be empty in other cases).
     "//build/config/sanitizers:deps",
   ]
+  ldflags = [ "-fprofile-instr-generate" ]

[lpromero, 2017/03/31 11:22:06]

I am not sure this is needed on the non test targets like here the Showcase app.

   bundle_deps = [ "//ios/showcase/core/resources" ]
   assert_no_deps = ios_assert_no_deps
 }
@@ -60,6 +61,7 @@ ios_eg_test("ios_showcase_egtests") {
     # asan mode (this target will be empty in other cases).
     "//build/config/sanitizers:deps",
   ]
+  ldflags = [ "-fprofile-instr-generate" ]

[lpromero, 2017/03/31 11:22:06]

Should I instead add to ios_eg_test template and use a variable to selectively
opt-into code coverage?

   bundle_deps = [ "//ios/showcase/core/resources" ]
   assert_no_deps = ios_assert_no_deps
 }