Document the Chromium for Chromium OS 'local_board'

When building Chromium for Chromium OS for Linux (linux-chromeos) there
is a choice of cros_boards, the local system and the Chromium sysroot.
At the moment linux-chromeos builds with the Chromium sysroot won't
run browser_tests on modern Debian systems due to an old version of
libffi. Since developer testing builds don't need to be protable anyway,
the Chromium sysroot is unnecessary. Document how to use the local
system as a solution to this.

Bug: chromium:1119849
Change-Id: Id5f7a41051367ae1f98cf48b1258680f6e11c9e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367736Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#800392}

docs/chromeos_build_instructions.md

@@ -68,6 +68,7 @@ or running `gn args out/Default`:
     is_debug = false           # Release build, runs faster.
     dcheck_always_on = true    # Enables DCHECK despite release build.
     enable_nacl = false        # Skips native client build, compiles faster.
+    use_sysroot = false        # Build for local machine instead of sysroot.
 
     # Set the following true to create a Chrome (instead of Chromium) build.
     # This requires a src-internal checkout.