Mirrors the change from 8e82c6ab1b%5E%21/
Also adds a new variable needed to build ToT ANGLE. This is because the toolchain change is unfortunately linked to rolling ANGLE at the same time, since the current ANGLE pull has build errors under VS 2017, but ToT hits internal compiler errors with the old toolchain, so they need to be updated in lockstep.
Part of fix for https://github.com/flutter/flutter/issues/48831
Changes to the canonical are only addition of a comment header and path-setting lines:
```
chrome_src = os.path.abspath(os.path.join(script_dir, os.pardir))
SRC_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
sys.path.insert(0, os.path.join(chrome_src, 'tools'))
```
