Increase link parallelism of linux build

This is re-land of
https://chromium-review.googlesource.com/891043

Changed not to increase link parallelism for chrome on chromeos.

Bug: 807145
Change-Id: I11c49d215704556eaa709af08ffe7226c3df529d
Reviewed-on: https://chromium-review.googlesource.com/892779
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533194}

build/toolchain/concurrent_links.gni

@@ -35,6 +35,8 @@ if (concurrent_links == -1) {
     _args = [ "--mem_per_link_gb=4" ]
   } else if (is_android && !is_component_build && symbol_level == 2) {
     _args = [ "--mem_per_link_gb=25" ]
+  } else if (is_linux && !is_chromeos && symbol_level == 1) {
+    _args = [ "--mem_per_link_gb=3" ]
   } else {
     _args = []
   }