serial: Simplify watcher usage in SerialPortUnderlyingSource

Rather than implementing a custom ArmWatcher method the ReadData method
(now named ReadDataOrArmWatcher) can call ArmOrNotify() whenever the
data pipe indicates that it should wait for more data.

The old code only had the advantage that it could catch the situation
where the data pipe became readable while the watcher was being armed.

This removes a number of DCHECKs which could fire in corner-case
situations.

Bug: 1256943
Change-Id: Id30e5372f53ff9e36cdc913b97738cc4aab04fa9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3206751
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#928961}