[ASH] Show IPH at onPageLoadFinished

The timing CustomTab history IPH shows feels way too early in the
tab lifecycle (almost immediately on url load). This is because
it uses TabObserver#onPageLoadStarted to trigger the UI.

Upon inspection, other IPHs observing tab events all rely on
TabObserver#onPageLoadFinished. This CL updates CustomTab history
IPH to observe load-finish signal instead.

Bug: crbug.com/324842054
Change-Id: I2db194d23779d412f7b4a4af0c299fd28be84461
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5980691
Reviewed-by: Sinan Sahin <sinansahin@google.com>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378431}