Added gn arg `msvc_enable_sse2` to enable SSE2 for MSVC builds

[Olivia Yingst]

Hi PDFium community,

A new gn arg `msvc_enable_sse2` has been added for PDFium and it's set to true by default (See implementation details and the related FreeType change here). This flag will by default enable SSE2 on MSVC builds, and it will be ignored on non-MSVC builds.

This flag allows the FreeType improvement on rendering Bezier quadratic arcs to be applied across all PDFium bots and keeps the font rendering results consistent across all Windows bots.

Best regards,

Hui Yingst