Add #! line to scripts/bootstrap.py. Indicate scripts/bootstrap.py should be run by python2. TBR=jmadill@chromium.org BUG=angleproject:2209 Change-Id: I2ccfee1407cd4ce457aa28a374ac202b8281d40b Reviewed-on: https://chromium-review.googlesource.com/862618 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Commit-Queue: Frank Henigman <fjhenigman@chromium.org>