Ensure manifest exists in Port._wpt_manifest.

third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py

Index: third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
diff --git a/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py b/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
index 0e184dc999446a7a291943d91812f2abb72af229..16efbbffa1f1f0578f88679a3bb59552e4aec38e 100644
--- a/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
+++ b/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
@@ -11,6 +11,7 @@ import optparse
 from webkitpy.common.net.git_cl import GitCL
 from webkitpy.layout_tests.models.test_expectations import BASELINE_SUFFIX_LIST
 from webkitpy.tool.commands.rebaseline import AbstractParallelRebaselineCommand
+from webkitpy.w3c.wpt_manifest import WPTManifest
 
 _log = logging.getLogger(__name__)
 
@@ -43,6 +44,9 @@ class RebaselineCL(AbstractParallelRebaselineCommand):
     def execute(self, options, args, tool):
         self._tool = tool
 
+        # TODO(qyearsley): Move this call to somewhere else.
+        WPTManifest.ensure_manifest(tool)
+
         unstaged_baselines = self.unstaged_baselines()
         if unstaged_baselines:
             _log.error('Aborting: there are unstaged baselines:')