Roll pdfium.

This brings in:
  12a9940 Check object type for objects used in the dictory of 'Index'
  9175bee Change the forced clear order of CPDF_DocPageData::Clear, and remove m_pDocument in CPDF_Color
  663af99 Fix uninitialized value in CPDF_ColorStateData
  6eee264 Revert of Fix uninitialized value in CPDF_ColorStateData
  a38d4c3 Fix uninitialized value in CPDF_ColorStateData
  0fb1b3d Fix un-initialized R G B value

TBR=inferno@chromium.org

Review URL: https://codereview.chromium.org/560403002

Cr-Commit-Position: refs/heads/master@{#294479}

DEPS

@@ -59,7 +59,7 @@ 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': 'edbf8ac5c5381a9e0dcc32032eafb823f571d7df',
+  'pdfium_revision': '12a9940905825e028bebc0bf8d3eddcc65b1933d',
   # 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.