ExternalDataUseObserver JNI integration

- ExternalDataUseObserver fetches matching rules over JNI
periodically.

- ExternalDataUseObserver buffers data use reports until
enough number of bytes have been buffered. Once enough
bytes have been buffered, data use report is sent over JNI.

- Field trial is used to control the value of various
constants.

BUG=540061

Review URL: https://codereview.chromium.org/1412813007

Cr-Commit-Position: refs/heads/master@{#358511}
3 files changed