Pull and build shaderc rather than use checked in lib/header

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1760493003

Committed: https://skia.googlesource.com/skia/+/7d4490ecfa4d974003f675ef004e77f85ecfadd2

Review URL: https://codereview.chromium.org/1760493003
7 files changed