Fix logging regression in WebfilesValidator
Fix logging regression in WebfilesValidator

Because BazelWorker overrides System.err globally on a per-invocation basis, we
need to make sure that Dagger constructs a new instance of WebfilesValidator
each time the command is invoked. This way the provideOutput() will get called
again and provide the magic PrintStream, whose data is intercepted, and encoded
in a proto that is sent back to Bazel.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146181902

bazelbuild/rules_closure,gramic/rules_closure,gramic/rules_closure,gramic/rules_closure,jart/rules_closure,bazelbuild/rules_closure,jart/rules_closure,bazelbuild/rules_closure,jart/rules_closure,jart/rules_closure,gramic/rules_closure,jart/rules_closure,bazelbuild/rules_closure