Increase coverage of telemetry's user_agent_unittest

Added a mobile and desktop testing in case the browser being tested has a
user agent that defaults to the tablet user agent.

BUG=421229

Review URL: https://codereview.chromium.org/640453002

Cr-Commit-Position: refs/heads/master@{#300349}

AUTHORS

@@ -96,6 +96,7 @@ Craig Schlenter <craig.schlenter@gmail.com>
 Daegyu Lee <na7jun8gi@gmail.com>
 Dai Chunyang <chunyang.dai@intel.com>
 Daniel Bomar <dbdaniel42@gmail.com>
+Daniel Imms <daniimms@amazon.com>
 Daniel Johnson <danielj41@gmail.com>
 Daniel Nishi <dhnishi@gmail.com>
 Daniel Shaulov <dshaulov@ptc.com>

tools/telemetry/telemetry/core/user_agent_unittest.py

@@ -6,11 +6,29 @@
 from telemetry.unittest import tab_test_case
 
 
-class UserAgentTest(tab_test_case.TabTestCase):
+class MobileUserAgentTest(tab_test_case.TabTestCase):
+  @classmethod
+  def CustomizeBrowserOptions(cls, options):
+    options.browser_user_agent_type = 'mobile'
+
+  def testUserAgent(self):
+    ua = self._tab.EvaluateJavaScript('window.navigator.userAgent')
+    self.assertEquals(ua, user_agent.UA_TYPE_MAPPING['mobile'])
+
+class TabletUserAgentTest(tab_test_case.TabTestCase):
   @classmethod
   def CustomizeBrowserOptions(cls, options):
     options.browser_user_agent_type = 'tablet'
 
   def testUserAgent(self):
     ua = self._tab.EvaluateJavaScript('window.navigator.userAgent')
     self.assertEquals(ua, user_agent.UA_TYPE_MAPPING['tablet'])
+
+class DesktopUserAgentTest(tab_test_case.TabTestCase):
+  @classmethod
+  def CustomizeBrowserOptions(cls, options):
+    options.browser_user_agent_type = 'desktop'
+
+  def testUserAgent(self):
+    ua = self._tab.EvaluateJavaScript('window.navigator.userAgent')
+    self.assertEquals(ua, user_agent.UA_TYPE_MAPPING['desktop'])