Resolve all but two VC++ build warnings in pdfium.

pdfium builds on Win32 have about 85 warnings (250 in the XFA
branch, totaling over 480 lines!), mostly from four lines in
a header file and a warning that should be disabled. This
change resolves all but two of them and turns on
warning-as-errors. Bugs have been filed for the two
remaining warnings:
https://code.google.com/p/pdfium/issues/detail?id=100
the 64-bit warnings:
https://code.google.com/p/pdfium/issues/detail?id=101
and the Linux warnings:
https://code.google.com/p/pdfium/issues/detail?id=102

The fix to the double->float truncation bugs will also
improve code-generation.

R=bo_xu@foxitsoftware.com, scottmg@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/792953005