Update some documentation for the binary_size tool

The documentation didn't include the crucial information
that generate_linker_map has to be set to true in gn for the tool
to work in Linux.

Bug: 716209
Change-Id: I62919e26a80a0b5d592f6b8953762e35575142d2
Reviewed-on: https://chromium-review.googlesource.com/c/1338085
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#609686}
3 files changed