Fix skiaserve crash when displaying batch bounds

In my test case, the first actual batch would combine with the clear
issued before auditing had been enabled. After the successful combine,
we would try to inform the audit manager, but it didn't know about the
batch, triggering an assert (and subsequent crash). Fix is ijust to
ensure that no prior batches are pending when we turn on auditing.

BUG=skia:
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Review URL: https://codereview.chromium.org/1894633002
1 file changed