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This change updates net_log_test_util's methods that retrieve strings
from NetLogEvent dictionaries to allow fetching a string from a nested
dictionary using the base::Value::GetStringPath syntax (for instance,
"inner.item" will fetch { "inner": { "item": "value" } }).

(This allows writing some slightly more concise test code in the child
CL crrev.com/c/2315641.)

R=eroman

Bug: None
Change-Id: Ie8552b9347e9bb147f12e1a57ca98a43b9216f88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354293
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798224}