UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36

Steps to reproduce the problem:
if `env` is located anywhere other than `/usr/bin/env` GN's bootstrap.py will fail.

What is the expected behavior?

What went wrong?
tools/gn/bootstrap/bootstrap.py fails because it call build/write_buildflag_header.py which uses a shebang `#!/usr/bin/env python`, however not all Unix-like operating systems use the `/usr` PREFIX.

Did this work before? No 

Chrome version: 54.0.2840.59  Channel: dev
OS Version: n/a
Flash Version: Shockwave Flash 23.0 r0

There is already a fix for windows only that if the command is not null and ends in .py it prepends sys.executable.  It should be safe to remove the is_win check and simply always prepend it to python scripts called by bootstrap.py.

https://chromium.googlesource.com/chromium/src/tools/gn/+/b26616d48e3801ac0539080850ee9c6a2a916b9f/bootstrap/bootstrap.py#41