[Autofill Assistant] Fix race condition while scrolling in Focus action.

Before this change, the Focus action usually scrolled smoothly elements
into view at 1/4 of the screen size. This sometimes failed, as right
after smooth scrolling started, scrollIntoViewIfNeeded is called which,
if it scrolls something, interrupts the smooth scrolling. The result is
that the element is sometimes left too low on the screen - though always
visible.

This change disables smooth scrolling, which avoids the race condition.
The cost of that is that we lose the nice scrolling animation.

Bug: b/132137331
Change-Id: I9f43e8a26cea6d6202262dd8b84a60efd45f9e8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599451Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658538}

components/autofill_assistant/browser/web_controller.cc

@@ -61,10 +61,9 @@ const char* const kScrollIntoViewScript =
     R"(function(node) {
     const rect = node.getBoundingClientRect();
     if (rect.height < window.innerHeight) {
-      window.scrollBy({top: rect.top - window.innerHeight * 0.25,
-        behavior: 'smooth'});
+      window.scrollBy({top: rect.top - window.innerHeight * 0.25});
     } else {
-      window.scrollBy({top: rect.top, behavior: 'smooth'});
+      window.scrollBy({top: rect.top});
     }
     node.scrollIntoViewIfNeeded();
   })";