Rename DocumentMarker::activeMatch() to IsActiveMatch()

What: Rename DocumentMarker::activeMatch() to IsActiveMatch()
Why: Adding "Is" makes it very clear that this is an accessor returning a boolean value, and aligns with other boolean predicates, e.g. isFocused() etc.
Outcome: Improve readability and code health

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

BUG=707867
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2789373003
Cr-Commit-Position: refs/heads/master@{#462190}
6 files changed