Revert "Revert of FileWriter should not use deprecated {,un}setPendingActivity (https://codereview.chromium.org/383103002/)"

This attempts to reland the patch with correct condition in |FileWriter::hasPendingActivity()|.
Before this patch, it said m_readyState != WRITING then it "hasPendingActivity". Which is obviously opposite.

TEST=fast/filesystem run with --enable-leak-detection does not leak
BUG=354845

Review URL: https://codereview.chromium.org/387253002

git-svn-id: svn://svn.chromium.org/blink/trunk@178021 bbb929c8-8fbe-4397-9dbb-9b2b20218538