Account for flings consumed by the InputRouterClient
WebView can consume flings that target non-root scrollably layers via
the InputRouterClient. Previously, we failed to account for these flings
in the InputRouter bookkeeping, leading to scenarios where the fling-end
notification would yield a negative active fling count.
Handle this case by also tracking flings that are consumed by the
client, with the implicit contract being that a fling end notification
should always accompany a "consumed" fling.
BUG=492274
Review URL: https://codereview.chromium.org/
1153143003
Cr-Commit-Position: refs/heads/master@{#331468}