Fix bisect-builds.py crash for non-official win64 builds

This patch fixes a bug that bisect-builds.py crashes when attempting to download non-official Win64 builds.

BUG=478255
TEST=python tools\bisect-builds.py -a win64 -g 415582 -b 416585

Review-Url: https://codereview.chromium.org/2348093002
Cr-Commit-Position: refs/heads/master@{#420544}

tools/bisect-builds.py

@@ -157,6 +157,8 @@ class PathContext(object):
       self._binary_name = 'Chromium.app/Contents/MacOS/Chromium'
     elif self.platform == 'win':
       self._listing_platform_dir = 'Win/'
+    elif self.platform == 'win64':
+      self._listing_platform_dir = 'Win_x64/'
 
   def GetASANPlatformDir(self):
     """ASAN builds are in directories like "linux-release", or have filenames