[build] Disable linker threading in LTO builds, as it causes intermittent link failures.

BUG=chromium:513074
R=bmeurer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1249273003

Cr-Commit-Position: refs/heads/master@{#29827}

build/toolchain.gypi

@@ -1149,7 +1149,9 @@
               }],
             ],
           }],
-          ['linux_use_gold_flags==1', {
+          # TODO(pcc): Re-enable in LTO builds once we've fixed the intermittent
+          # link failures (crbug.com/513074).
+          ['linux_use_gold_flags==1 and use_lto==0', {
             'target_conditions': [
               ['_toolset=="target"', {
                 'ldflags': [