This updates //build/install-build-deps.sh to use `python3` instead
of `python`, and updates //docs/linux/build_instructions.md to
mention that you need python3 installed ahead of time rather than
Python 2 (since we'll install Python 2 via install-build-deps).
Bug: 112471
Change-Id: I8ba08d65d26ac52a05bce248c319861b0a3156f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2845609
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#875304}