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This fixes a typo introduced r419404, where the GN label for
webapk_shell_apk_junit_tests had one too many leading slashes.

TBR=brettw@chromium.org
NOTRY=true
BUG=555273

Review-Url: https://codereview.chromium.org/2349993002
Cr-Commit-Position: refs/heads/master@{#419409}