Merge to XFA: Add new public APIs to find the z-order for links and widgets.

Merge to XFA: Add new public APIs to find the z-order for links and widgets.

- Implement FPDFLink_GetLinkZOrderAtPoint().
- Implement FPDFPage_FormFieldZOrderAtPoint().
- Mark FPDPage_HasFormFieldAtPoint() as deprecated.
- Modify CPDF_LinkList and CPDF_InterForm to support new APIs.
- Clean up dead code in CPDF_LinkList and CPDF_InterForm.

BUG=chromium:515837
R=jun_fang@foxitsoftware.com, tsepez@chromium.org

Review URL: https://codereview.chromium.org/1278053004 .

(cherry picked from commit 24fbf134d43a7ec4226de3db601f0a617bbe428b)

Committed: https://pdfium.googlesource.com/pdfium/+/bdf72c353af5b8a34ef5efdeddfff36d2089a158

[Lei Zhang, 2015-08-15 02:23:59]

TBR

https://codereview.chromium.org/1295893005/diff/1/core/src/fpdfdoc/doc_form.cpp
File core/src/fpdfdoc/doc_form.cpp (left):

https://codereview.chromium.org/1295893005/diff/1/core/src/fpdfdoc/doc_form.c...
core/src/fpdfdoc/doc_form.cpp:1332: if (pControl == NULL) {
conflicted here.

https://codereview.chromium.org/1295893005/diff/1/fpdfsdk/src/fpdfdoc.cpp
File fpdfsdk/src/fpdfdoc.cpp (right):

https://codereview.chromium.org/1295893005/diff/1/fpdfsdk/src/fpdfdoc.cpp#new...
fpdfsdk/src/fpdfdoc.cpp:191: CPDFXFA_Page* pXFAPage = (CPDFXFA_Page*)page;
conflicted here.

https://codereview.chromium.org/1295893005/diff/1/public/fpdf_doc.h
File public/fpdf_doc.h (right):

https://codereview.chromium.org/1295893005/diff/1/public/fpdf_doc.h#newcode194
public/fpdf_doc.h:194: // Notes:
conflicted here.

[Lei Zhang, 2015-08-15 02:24:12]

Committed patchset #2 (id:20001) manually as
bdf72c353af5b8a34ef5efdeddfff36d2089a158 (presubmit successful).