As part of eliminating the Flutter buildroot (#67373), we are moving all third-party dependencies from //third_party to //flutter/third_party.

Steps:

• Buildroot: Migrate BUILD.gn to build/secondary/flutter/third_party/BUILD.gn Move //third_party/glfw to //flutter/third_party/glfw buildroot#777
• Engine: Update DEPS and references to use new location/targets Move //third_party/glfw to //flutter/third_party/glfw engine#46733

This is an abridged form of a longer, stepwise migration that we may use for more complex future migrations:

• Buildroot: Copy BUILD.gn to build/secondary/flutter/third_party/BUILD.gn
• Engine: Update DEPS and references to use new location
• Buildroot: Eliminate old BUILD.gn from build/secondary/third_party/BUILD.gn
• Engine: Eliminate DEPS references to old location

Part of #67373