7591 Roll buildroot to b4d21cb2a64d63218c8d99533d9c14e99201e8d8 (cla: yes)

7602 Update buildtools to c9e5400c9e03a0cfb7313d14fde38525399a7715 (cla: yes)

7610 Provide public api to allow FlutterEngine related context to be destoryed (affects: engine, cla: yes, customer: gold, platform-ios)

7611 Update license to sync with flutter/flutter (cla: yes)

7617 Allow the engine to redirect traces to systrace via settings. (cla: yes)

7621 Improve PathMetrics (cla: yes)