Roll pdfium from f8105c5 to f731837.

Pulls in the following changes:

 f731837 - Fix v8 external snapshot code for Windows.
 7900739 - Merge to master: Fix include paths for pdfium_unittests under chromium checkout
 476cd69 - Add APIs for getting bookmarks and named destinations.

BUG=421063, 439661

Review URL: https://codereview.chromium.org/850043003

Cr-Commit-Position: refs/heads/master@{#311489}

DEPS

@@ -66,7 +66,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.
-  'pdfium_revision': 'f8105c665856863ad95da37fee6c12b98b953e2c',
+  'pdfium_revision': 'f731837225b825e154d6b1664160ed6f70d451ce',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling openmax_dl
   # and whatever else without interference from each other.