Rename DocumentMarker::setActiveMatch() to setIsActiveMatch()

Suggested by @yosin here:
https://codereview.chromium.org/2801483002#msg20

What: Rename DM::setActiveMatch() to setIsActiveMatch()
Why: Align with the rename of accessor method activeMatch() to IsActiveMatch() in https://codereview.chromium.org/2789373003
Outcome: Improve readability and code health

BUG=707867

Review-Url: https://codereview.chromium.org/2802533002
Cr-Commit-Position: refs/heads/master@{#462198}
3 files changed