Linux: Enable multi-threading with the gold linker.

BUG=161942

Review URL: https://codereview.chromium.org/894463003

Cr-Commit-Position: refs/heads/master@{#314991}

build/common.gypi

@@ -4405,10 +4405,8 @@
                   # https://groups.google.com/a/chromium.org/group/chromium-dev/browse_thread/thread/281527606915bb36
                   # Only apply this to the target linker, since the host
                   # linker might not be gold, but isn't used much anyway.
-                  # TODO(raymes): Disable threading because gold is frequently
-                  # crashing on the bots: crbug.com/161942.
-                  # '-Wl,--threads',
-                  # '-Wl,--thread-count=4',
+                  '-Wl,--threads',
+                  '-Wl,--thread-count=4',
                 ],
               }],
             ],

build/config/compiler/BUILD.gn

@@ -380,10 +380,8 @@ config("compiler") {
       # https://groups.google.com/a/chromium.org/group/chromium-dev/browse_thread/thread/281527606915bb36
       # Only apply this to the target linker, since the host
       # linker might not be gold, but isn't used much anyway.
-      # TODO(raymes): Disable threading because gold is frequently
-      # crashing on the bots: crbug.com/161942.
-      #"-Wl,--threads",
-      #"-Wl,--thread-count=4",
+      "-Wl,--threads",
+      "-Wl,--thread-count=4",
     ]
   }