Upgrade Blink to milliseconds-based last modified filetimes, part 6.

Upgrade Blink to milliseconds-based last modified filetimes, part 6.

Have content::FileInfoToWebFileInfo() no longer fill in
WebFileInfo::modificationTimeMS; it is now unused by Blink.

Once landed, this lets Blink remove the WebFileInfo field entirely.

This is part 6 in the following patch series,

1: [blink] add WebFileInfo::modificationTimeMS [ https://codereview.chromium.org/873723004/ ]
2: [chromium] fill in modificationTimeMS [ https://codereview.chromium.org/884413002/ ]
3: [blink] make use of modificationTimeMS [ https://codereview.chromium.org/884393002/ ]
4: [chromium] set modificationTime to something msec-based [ https://codereview.chromium.org/862203003/ ]
5: [blink] switch to modificationTime [ https://codereview.chromium.org/882343002/ ]
6: [chromium] *this one* [ https://codereview.chromium.org/890523002/ ]
7: [blink] remove modificationTimeMS [ https://codereview.chromium.org/869613005/ ]

R=jochen
BUG=451747

Review URL: https://codereview.chromium.org/890523002

Cr-Commit-Position: refs/heads/master@{#315545}