Drop 64 character limit in DeviceManagementRequest |agent| parameter

The HTTP POST |agent| parameter sent with a DeviceManagementRequest is
limited to 64 characters. After the git migration, Chrome includes its
git revision in the |agent|, making it longer than 64 characters.

This CL removes the arbitrary 64 character limit. This solution should be
considered temporary, until we have verified that the DMServer can handle
|agents| longer than 64 characters. If not, we will restore this limit and
change the construction of the string so that it never exceeds 64
characters.

BUG=406948
TEST=Browser tests no longer fail when the |agent| string is too long

Review URL: https://codereview.chromium.org/500013002

Cr-Commit-Position: refs/heads/master@{#291672}

chrome/browser/policy/test/policy_testserver.py

@@ -278,8 +278,7 @@ class PolicyRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
     # device_management_backend.proto.
     if (self.GetUniqueParam('devicetype') != '2' or
         self.GetUniqueParam('apptype') != 'Chrome' or
-        len(self.GetUniqueParam('deviceid')) >= 64 or
-        len(self.GetUniqueParam('agent')) >= 64):
+        len(self.GetUniqueParam('deviceid')) >= 64):
       return (400, 'Invalid request parameter')
     if request_type == 'register':
       response = self.ProcessRegister(rmsg.register_request)

components/policy/proto/device_management_backend.proto

@@ -643,7 +643,7 @@ message DeviceStateRetrievalResponse {
 //   * devicetype: MUST BE "1" for Android or "2" for Chrome OS.
 //   * apptype: MUST BE Android or Chrome.
 //   * deviceid: MUST BE no more than 64-char in [\x21-\x7E].
-//   * agent: MUST BE no more than 64-char long.
+//   * agent: MUST BE a string of characters.
 // * HTTP Authorization header MUST be in the following formats:
 //   * For register and ping requests
 //     Authorization: GoogleLogin auth=<auth cookie for Mobile Sync>