[reader mode] Adjust default triggering heuristic values

The current defaults are too noisy, adjusting them with a little trial
and error to something a little less eager.

Bug: None
Change-Id: Id8effaf52a0b3cf01889c9a181dd5b411089decd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6981644
Auto-Submit: Brandon Wylie <wylieb@google.com>
Reviewed-by: Evan Luo <evanluo@google.com>
Commit-Queue: Evan Luo <evanluo@google.com>
Cr-Commit-Position: refs/heads/main@{#1520528}