[iOS] Fix egtest for zoom text button

Text zoom should not be available on iPad unless `WebPageTextZoomIPad`
is enabled, which is currently not the case for eg tests. However, this
test always passes on iPad due to a bug, which is uncovered when we
disable website following on the tests (removing a row from settings.)

The bug was that on iPad, the disabled "Zoom Text..." button is not
visible in Settings, unless the user scrolls it. However, a wrong scroll
view is being used when the test attempts to search the button by
scrolling. As a result, the action
"selectElementWithMatcher:usingSearchAction:onElementWithMatcher"
returns a `nil` element, and that coincidentally matches the matcher
"grey_not(grey_accessibilityTrait(UIAccessibilityTraitNotEnabled))."

This CL updates this test so that we always make sure the "Zoom Text
..." button is visible, but would be disabled on iPad.

Bug: N/A
Change-Id: Ic164e67d15bae67d7be8d9d4e4d4e5654265b697
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6506144
Reviewed-by: Duong Dac <ddac@chromium.org>
Commit-Queue: Duong Dac <ddac@chromium.org>
Commit-Queue: Ginny Huang <ginnyhuang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1455056}