Add ability to specify a different base dir path to record_wpr

Motivation:
Skia has its own set of page sets that are run on our bots. They live in a
directory within Skia's repository. A similar change is going to be made
to skpicture_printer in https://codereview.chromium.org/665193002/

BUG=

Review URL: https://codereview.chromium.org/674523002

Cr-Commit-Position: refs/heads/master@{#301140}

tools/telemetry/telemetry/page/record_wpr.py

@@ -123,7 +123,12 @@ class WprRecorder(object):
     self._AddCommandLineArgs()
     self._ParseArgs(args)
     self._ProcessCommandLineArgs()
-    self._page_set = self._GetPageSet(base_dir, target)
+
+    if self._options.page_set_base_dir:
+      page_set_base_dir = self._options.page_set_base_dir
+    else:
+      page_set_base_dir = base_dir
+    self._page_set = self._GetPageSet(page_set_base_dir, target)
 
   @property
   def options(self):
@@ -144,6 +149,8 @@ class WprRecorder(object):
     return results_options.CreateResults(benchmark_metadata, self._options)
 
   def _AddCommandLineArgs(self):
+    self._parser.add_option('--page-set-base-dir', action='store',
+                            type='string')
     page_runner.AddCommandLineArgs(self._parser)
     if self._benchmark is not None:
       self._benchmark.AddCommandLineArgs(self._parser)

tools/telemetry/telemetry/page/record_wpr_unittest.py

@@ -164,6 +164,20 @@ class RecordWprUnitTests(tab_test_case.TabTestCase):
     self.assertEqual(set(mock_benchmark.mock_page_set.pages),
                      results.pages_that_succeeded)
 
+  def testPageSetBaseDirFlag(self):
+    flags = [
+       '--page-set-base-dir', self._test_data_dir,
+       '--mock-benchmark-url', self._url,
+       '--browser', self._browser.browser_type,
+    ]
+    mock_benchmark = MockBenchmark()
+    wpr_recorder = record_wpr.WprRecorder(
+        'non-existent-dummy-dir', mock_benchmark, flags)
+    results = wpr_recorder.CreateResults()
+    wpr_recorder.Record(results)
+    self.assertEqual(set(mock_benchmark.mock_page_set.pages),
+                     results.pages_that_succeeded)
+
   def testCommandLineFlags(self):
     flags = [
         '--page-repeat', '2',