[Build] Allow GN flag generate_linker_map=true for target="chromeos".

SuperSize requires GN flag generate_linker_map = true, but previously
the flag is available only for Android or Linux. This CL makes the flag
usable for ChromeOS builds as well, to enable SuperSize usage.

Bug: 1084580
Change-Id: Ibfd13e7433a6b4cc6af43a8fddcb7510ea757e5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218699
Commit-Queue: Samuel Huang <huangs@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772397}

build/toolchain/toolchain.gni

@@ -31,8 +31,10 @@ if (generate_linker_map) {
       is_official_build,
       "Linker map files should only be generated when is_official_build = true")
   assert(current_os == "android" || current_os == "linux" ||
-             target_os == "android" || target_os == "linux",
-         "Linker map files should only be generated for Android and Linux")
+             target_os == "android" || target_os == "linux" ||
+             target_os == "chromeos",
+         "Linker map files should only be generated for Android, Linux, " +
+             "or ChromeOS.")
 }
 
 declare_args() {