Clarifies that gclient sync should happen before codegen to avoid using
stale hashes on upload.

Bug: angleproject:3227
Change-Id: I7777458b6fa659a07307ad1d845ac7d332afa8f2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1934313Reviewed-by: Ethan Lee <flibitijibibo@gmail.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>