This is a preparation CL for implementing toggle annotation button.
FPDF_ANNOT flag is needed to draw non-widget type PDF annotations, such
as text, highlight, underline, etc. Same flag needs to be passed to
FPDF_RenderPageBitmap_Start and FPDF_FFLDraw in order to display /
hide popup window associated to non-widget annotation on mouse-hover.
BUG=642982
Review-Url: https://codereview.chromium.org/2328893002
Cr-Commit-Position: refs/heads/master@{#417745}