Domain Reliability: Make context upload test cases less brittle
Domain Reliability: Make context upload test cases less brittle

Individually examine the fields of the uploaded beacon instead of
comparing the JSONified report to a static test string.

BUG=

Review URL: https://codereview.chromium.org/1511243003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#364401}

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