Basic APIs and tests for extracting and setting annotation paths

1. Added APIs for retrieving existing annotation paths and setting
annotation paths.
    * Added an embedder test testing all the new functions.

Bug=pdfium:737

Change-Id: Ic451bcd3be488261baf2182549c4238b887b219e
Reviewed-on: https://pdfium-review.googlesource.com/6676
Commit-Queue: Jane Liu <janeliulwq@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/fpdfview.cpp b/fpdfsdk/fpdfview.cpp
index ccd487a..1c282a7 100644
--- a/fpdfsdk/fpdfview.cpp
+++ b/fpdfsdk/fpdfview.cpp
@@ -309,6 +309,10 @@
   return static_cast<CPDF_PathObject*>(page_object);
 }
 
+CPDF_PageObject* CPDFPageObjectFromFPDFPageObject(FPDF_PAGEOBJECT page_object) {
+  return static_cast<CPDF_PageObject*>(page_object);
+}
+
 CFX_DIBitmap* CFXBitmapFromFPDFBitmap(FPDF_BITMAP bitmap) {
   return static_cast<CFX_DIBitmap*>(bitmap);
 }