Add preprocessor tags in Presentation Mode code

This PR is the fifth in a series of patches that will improve the presentation mode implementation. (The previous ones were #3752, #3763, #3774 and #3777.)

This patch adds preprocessor tags in the Presentation Mode code, so that we stop including WebKit and Microsoft specific code in the Mozilla versions (builds created with `node make {firefox, mozcentral, b2g}`) of PDF.js.