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This fixes an existing TODO in url_pattern_index to implement case-insensitive
matching. To do so:
  - Lower case the stored n grams. These n grams are used to find prospective
    matches for a particular url.
  - Modify URLPattern to handle case-insensitive matching. This involves
    lower-casing the url pattern and the url, before comparing them.

BUG=767605

Cq-Include-Trybots: master.tryserver.chromium.perf:obbs_fyi
Change-Id: I1377c216d61392c508280141d0d947a47a3ee494
Reviewed-on: https://chromium-review.googlesource.com/1212183Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590118}