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This CL adds "regexFilter" to schema of declarative net request rules. These
rules are also serialized to flatbuffer. Tests are added to check rule
validation and serialization logic.

In subsequent CLs matching for these rules will be implemented.

Skipping presubmits due to crbug.com/956368.

BUG=974391
Doc=https://docs.google.com/document/d/1mRErUMII_gSSPaHmxyn31UOYWUaZLj0xOaezekxD2-Y/edit?usp=sharing (Internal)
NOPRESUBMIT=true

Change-Id: I356e57ed25ce187f145306e50ca169369f09262e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869556
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709187}