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/1338085Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#609686}

build/toolchain/toolchain.gni

@@ -23,8 +23,6 @@ declare_args() {
   use_xcode_clang = is_ios && is_official_build
 
   # Used for binary size analysis.
-  # Currently disabled on LLD because of a bug (fixed upstream).
-  # See https://crbug.com/716209.
   generate_linker_map = is_android && is_official_build
 
   # Use absolute file paths in the compiler diagnostics and __FILE__ macro

tools/binary_size/README.md

@@ -65,6 +65,8 @@ Supports Android and Linux (although Linux
    * Map files contain some unique pieces of information compared to `nm`
       output, such as `** merge strings` entries, and some unnamed symbols
       (which although unnamed, contain the `.o` path).
+   * Generated in `is_official_build=true` builds if `generate_linker_map` is
+     true. In official builds on Android generate_linker_map is true by default.
 1. `.o` files are mapped to `.cc` files by parsing `.ninja` files.
    * This means that `.h` files are never listed as sources. No information
      about inlined symbols is gathered.

tools/binary_size/libsupersize/archive.py

@@ -1507,8 +1507,8 @@ def DeduceMainPaths(args, parser):
       map_path += '.gz'
     if not os.path.exists(map_path):
       parser.error('Could not find .map(.gz)? file. Ensure you have built with '
-                   'is_official_build=true, or use --map-file to point me a '
-                   'linker map file.')
+                   'is_official_build=true and generate_linker_map=true, or '
+                   'use --map-file to point me a linker map file.')
 
   linker_name = _DetectLinkerName(map_path)
   logging.info('Linker name: %s' % linker_name)