Use X11 headers only when using X11.
Use X11 headers only when using X11.

This was causing problems when building chromium on Linux platforms with no x11 and Ozone enabled. https://codereview.chromium.org/817023005/diff/40001/content/common/BUILD.gn
made changes so that we run the version in content_common with a va_stub_header.fragment always.

Review URL: https://codereview.chromium.org/860163003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#312563}

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