Fix re-layout loop from leaked NTP global layout listener in native input - #9787

Task/Issue URL: https://app.asana.com/1/137249556945/project/1204912272578138/task/1218383698693718?focus=true
Tech Design URL (if applicable):
API Proposals URL(s) (if applicable):

Description

On a Galaxy Tab S9+ (doesn't reproduce the same on all devices), holding backspace in the native input widget became ~3x slower (about 220 ms per character instead of about 66 ms) after switching tabs a few times via the tab switcher. Swiping between tabs does not reproduce it.

Cause: NativeInputLayoutCoordinator.configureContentOffset registered an OnGlobalLayoutListener on ntpContentView.viewTreeObserver and removed it via the same getter on widget detach. When a tab is opened from the tab switcher while the previous tab had the input open, the outgoing tab's BrowserTabViewModel#onViewVisible is triggered, showing the native input and registering the global layout listener, split second before its torn down for the selected tab instance. By the time the widget's detach callback runs, the NTP content is already detached and the getter returns a different observer, so the removal is a no-op. The listener leaks, once per switch.

Each leaked session (widget not shown, wants padding 0) then fights the live session (wants the widget offset) on every global layout pass, both trying to setPadding which posts a requestLayout. This keeps the main thread busy relayouting on every frame, so each keyboard event has to wait for a free frame before the app can handle it. Held backspace, which fires many events per second, is where this shows up most. Nothing visibly redraws, so it looks like a slow text field.

Fix: capture the observer instance at registration and remove from that instance.

Steps to test this PR

Held backspace after tab switches

• Fresh app instance, single tab
• Run a search query
• Open a new tab
• Switch between the search tab and the new tab multiple times through the tab switcher activity
• Type text in the new tab, hold backspace: deletion speed matches a fresh single-tab session

No regression in content offset

• Focus the omnibar on NTP and in a loaded page with both top and bottom omnibar: content stays offset below/above the widget, no jump after the enter animation
• Close the input and reopen after a tab swipe: no stacked or leftover padding