Add got_revision key to pdfium buildbot config.

This is needed for run_presubmit.py to work.

BUG=pdfium:971

Change-Id: Ib598fbfd6eab3c0b7628301eb3bd54c548a4885e
Reviewed-on: https://chromium-review.googlesource.com/898039
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

recipes/recipe_modules/gclient/config.py

@@ -245,6 +245,8 @@ def pdfium(c):
   soln = c.solutions.add()
   soln.name = 'pdfium'
   soln.url = 'https://pdfium.googlesource.com/pdfium.git'
+  m = c.got_revision_mapping
+  m['pdfium'] = 'got_revision'
 
 @config_ctx()
 def mojo(c):