Simplify UnguessableToken::ToString, don't wrap it in ().

This change migrates unguessable tokens' serialization format from
`(790345...265EE7)` to `7903415...265EE7`.

When unguessable token is used for instrumentation and tracing, it can be used
as a handle. Having non-URL characters such as punctuation in the serialized
form of the token limits its use and requires it to be escaped / post-processed.

If wrapping in () is not essential to the token, I'm suggesting that we drop it.

Bug: 778263
Change-Id: Ib88c15c478f6dca89425f272e35893dd399f91fd
Reviewed-on: https://chromium-review.googlesource.com/845146
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526627}

diff --git a/base/unguessable_token_unittest.cc b/base/unguessable_token_unittest.cc
index 0158257..287fca31 100644
--- a/base/unguessable_token_unittest.cc
+++ b/base/unguessable_token_unittest.cc

@@ -80,13 +80,14 @@
 
 TEST(UnguessableTokenTest, VerifyToString) {
   UnguessableToken token = UnguessableToken::Deserialize(0x123, 0xABC);
-  std::string expected = "(0000012300000ABC)";
+  std::string expected = "0000012300000ABC";
 
   EXPECT_EQ(expected, token.ToString());
 
+  std::string expected_stream = "(0000012300000ABC)";
   std::stringstream stream;
   stream << token;
-  EXPECT_EQ(expected, stream.str());
+  EXPECT_EQ(expected_stream, stream.str());
 }
 
 TEST(UnguessableTokenTest, VerifySmallerThanOperator) {