Roll PDFium to bring in the following changes:

615082d  Remove OpenJPEG header generator binary from PDFium build.
f3823b2  Add embedder test for fix to bug_452455.

Review URL: https://codereview.chromium.org/899593003

Cr-Commit-Position: refs/heads/master@{#314380}

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': 'b3a788e5e37955620b26be5b6e3048d37b605e00',
+  'pdfium_revision': '615082de70c7fc18d46d0d1a03b62d0d76b1daa9',
   # 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.