extensions: Add missing xdisplaycheck dep to extensions_unittests.
extensions: Add missing xdisplaycheck dep to extensions_unittests.

This is a runtime dependency that is necessary in order to run
extensions_unittests under isolate.

This came in while doing https://codereview.chromium.org/1238453003/.

BUG=None
TEST=in order to reproduce this, change a source file (a _test.cc) that
is run under extensions_unittests in the clobber build (rm -rf out/).
The analyze step in the linux clobber bot should detect the source
change and trigger this.
R=rdevlin.cronin@chromium.org,maruel@chromium.org,rockot@chromium.org

Review URL: https://codereview.chromium.org/1228353006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339104}

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