[MessageLoopForIO] Rename variables after standardizing MessagePumpForIO::FdWatchController class name.

Missed those in associated migration.

Alleviates reading confusion I experienced while reviewing
https://chromium-review.googlesource.com/c/chromium/src/+/695914

[email protected], [email protected]

Bug: 825327
Change-Id: I2f9befb037d7931ef4471da660c3f9eeb91e8ecd
Reviewed-on: https://chromium-review.googlesource.com/1054414
Reviewed-by: Scott Graham <[email protected]>
Reviewed-by: François Doray <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#558040}
3 files changed