Add content ID scheme to //url/url_constants.*

This CL adds the content ID scheme to //url/url_constants.*. This scheme is
specified in https://tools.ietf.org/html/rfc2392. The concrete motivation for
this CL is that an upcoming CL needs to reference the content ID scheme in
order to correctly handle navigations within MHTML documents when using
browser-side navigation.

BUG=623553

Review-Url: https://codereview.chromium.org/2098723003
Cr-Commit-Position: refs/heads/master@{#402185}

url/gurl_unittest.cc

@@ -660,6 +660,9 @@ TEST(GURLTest, IsStandard) {
 
   GURL c("foo://bar/baz");
   EXPECT_FALSE(c.IsStandard());
+
+  GURL d("cid:bar@baz");
+  EXPECT_FALSE(d.IsStandard());
 }
 
 TEST(GURLTest, SchemeIsHTTPOrHTTPS) {

url/url_constants.cc

@@ -11,6 +11,7 @@ const char kAboutBlankURL[] = "about:blank";
 const char kAboutScheme[] = "about";
 const char kBlobScheme[] = "blob";
 const char kContentScheme[] = "content";
+const char kContentIDScheme[] = "cid";
 const char kDataScheme[] = "data";
 const char kFileScheme[] = "file";
 const char kFileSystemScheme[] = "filesystem";

url/url_constants.h

@@ -17,6 +17,7 @@ URL_EXPORT extern const char kAboutScheme[];
 URL_EXPORT extern const char kBlobScheme[];
 // The content scheme is specific to Android for identifying a stored file.
 URL_EXPORT extern const char kContentScheme[];
+URL_EXPORT extern const char kContentIDScheme[];
 URL_EXPORT extern const char kDataScheme[];
 URL_EXPORT extern const char kFileScheme[];
 URL_EXPORT extern const char kFileSystemScheme[];