Download or update Mac toolchain for pdfium
Download or update Mac toolchain for pdfium

Add Mac toolchain update logic to check whether toolchain should be
downloaded. For googlers, it should be downloaded by default. This fix
the current sdk directory couldn't be found problem.

For googlers who want to opt out of using toolchain, set "use_system_xcode
= true" in gn config.

Review-Url: https://codereview.chromium.org/2515833002

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium