Add /clank to .gitignore

In the past Chrome for Android developers have excluded this directory using
src/.git/info/exclude or simply ignored it.

It's time to add it to .gitignore, since we already have
/third_party/android_tools_internal in there, and the codename isn't public
(https://plus.google.com/+FrancoisBeaufort/posts/AJwwscC1KxN).

Review URL: https://codereview.chromium.org/815353004

Cr-Commit-Position: refs/heads/master@{#311484}

.gitignore

@@ -135,6 +135,7 @@ v8.log
 /chrome/unit_tests_run.xml
 /chrome/web_ui_mojo_bindings.xml
 /chrome_elf/chrome_elf_resources.xml
+/clank
 /cloud_print/cloud_print_version_resources.xml
 /components/chrome_settings_proto_generated_compile.xml
 /components/cloud_policy_proto_generated_compile.xml