* build: define compile time features with buildflag header
* refactor: switch to BUILDFLAG(ENABLE_DESKTOP_CAPTURER)
* refactor: switch to BUILDFLAG(ENABLE_RUN_AS_NODE)
* refactor: switch to BUILDFLAG(ENABLE_OSR)
* refactor: switch to BUILDFLAG(ENABLE_VIEW_API)
* refactor: switch to BUILDFLAG(ENABLE_PEPPER_FLASH)
* refactor: switch to BUILDFLAG(OVERRIDE_LOCATION_PROVIDER)
* refactor: switch to BUILDFLAG(ENABLE_PDF_VIEWER)