Fix //content/plugin ax_enums gn build flake (take 2)
Fix //content/plugin ax_enums gn build flake (take 2)

Flake observerd with target_os="android" and enable_plugins=true,
probably can happen on other platforms as well.
//content/plugin/plugin_thread.cc ends up including the generated
ax_enums header so needs a dependency.

This is a retry of crrev.com/1343883003 with a direct ax_gen dep instead
of going via //content/public/common to avoid a dependency cycle in
component builds.

R=ananta@chromium.org, jam@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1344883002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#349228}

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