Add media router common classes and unittests.

chrome/browser/media/router/media_source.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/media/router/media_source.h"
+
+#include "base/strings/string_util.h"
+#include "base/strings/stringprintf.h"
+#include "url/gurl.h"
+
+namespace media_router {
+
+namespace {
+
+// See: https://www.ietf.org/rfc/rfc3406.txt
+const char* kTabMediaUrnPrefix = "urn:x-org.chromium.media:source:tab";

[xhwang, 2015/03/05 22:28:50]

nit. We prefer 

const char kFoo[]

to 

const char* kFoo

In theory, the latter allows you to assign a new value to kFoo, which is not
desired.

[Kevin M, 2015/03/06 23:12:45]

Good point, done.

+const char* kDesktopMediaUrnPrefix = "urn:x-org.chromium.media:source:desktop";
+const char* kCastUrnPrefix = "urn:x-com.google.cast:application:";

[xhwang, 2015/03/05 22:28:50]

It's confusing that some prefix ends with ":" and some doesn't.

[Kevin M, 2015/03/06 23:12:45]

It's totally media type dependent here.

For tab, the media ID is the concatenated form "tabX" where X is the ID of a
tab.
For desktopMediaUrnPrefix, it's not meant to be prepended to anything, so that's
a misnomer. I'll fix it.
For CastURN, the cast app ID is used as-is and not concatenated with any other
tokens.

[xhwang, 2015/03/09 17:37:30]

sg, add a comment? It's easy for people to omit this.

[Kevin M, 2015/03/09 23:32:06]

Done.

+
+}  // namespace

[xhwang, 2015/03/05 22:28:50]

not needed. const variables are static by default.

http://stackoverflow.com/questions/177437/const-static

[Kevin M, 2015/03/06 23:12:45]

I'm not sure about that. const variables can definitely be referenced from
outside the module via use of externs.
http://stackoverflow.com/questions/2190919/mixing-extern-and-const

The SO article you shared applies to const statics, but these aren't static.

[xhwang, 2015/03/09 17:37:30]

I am talking about 

const char kFoo[] = "..."

without extern and without the anonymous namespace. In that case kFoo is
"static" by default which means it can only be referenced from this module. So
the anonymous namespace doesn't do anything here.

An example of a previous discussion:
https://codereview.chromium.org/24649002/diff/16001/cc/trees/thread_proxy.cc#...

[Kevin M, 2015/03/09 23:32:06]

OK, done.

+
+MediaSource ForTabMediaSource(int tab_id) {
+  return base::StringPrintf("%s:%d", kTabMediaUrnPrefix, tab_id);
+}
+
+MediaSource ForDesktopMediaSource() {
+  return std::string(kDesktopMediaUrnPrefix);
+}
+
+// TODO(mfoltz): Remove when the TODO in
+// MediaSourceManager::GetDefaultMediaSource is resolved.
+MediaSource ForCastAppMediaSource(const std::string& app_id) {
+  return kCastUrnPrefix + app_id;
+}
+
+MediaSource ForPresentationUrl(const std::string& presentation_url) {
+  return presentation_url;
+}
+
+bool IsMirroringMediaSource(const MediaSource& source) {
+  return StartsWithASCII(source, kDesktopMediaUrnPrefix, true) ||
+    StartsWithASCII(source, kTabMediaUrnPrefix, true);
+}
+
+bool IsValidMediaSource(const MediaSource& source) {
+  if (IsMirroringMediaSource(source) ||
+      StartsWithASCII(source, kCastUrnPrefix, true)) {
+    return true;
+  }
+  GURL url(source);
+  return url.is_valid() && url.SchemeIsHTTPOrHTTPS();
+}
+
+}  // namespace media_router