Mark //components/crash/core/common:unit_tests as testonly.
Mark //components/crash/core/common:unit_tests as testonly.

Followup to http://crrev.com/1358483002 that turned mac_chromium_gn_rel red
(I don't know how the CL landed through the CQ).

Fixes the following error:

  ERROR at //components/crash/core/common/BUILD.gn:23:1: Test-only dependency not allowed.
  source_set("unit_tests") {
  ^-------------------------
  //components/crash/core/common:unit_tests
  which is NOT marked testonly can't depend on
  //testing/gtest:gtest
  which is marked testonly. Only targets with "testonly = true"

BUG=522955
TBR=rsesek@chromium.org

Review URL: https://codereview.chromium.org/1348353004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#350149}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk