[headless] Avoid compiling files that will not be used

In component builds headless_shell_lib compiled
lib/browser/headless_content_browser_client.cc which referred to
a lot of other code that was not compiled. Thanks to a linker
feature/quirk, the whole object file was ignored and nothing
bad happened. In jumbo build experiments the linker couldn't
do dead object file removal and the result was a lot of linkage
errors.

Bug: 883727
Change-Id: I3ef9f69cbf61ddccdb55d0929963ad33a6fb442b
Reviewed-on: https://chromium-review.googlesource.com/1226615
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591683}
1 file changed