[iOS] Fix incorrect default CRWWebViewScrollViewProxy properties

CRWWebViewScrollViewProxy initializes itself with a base UIScrollView
so any properties set on it before a real web view scroll view is set
are used (see the comment in CRWWebViewScrollViewProxy -init). Then,
when another scroll view is added to the proxy, all properties are
copied over from the stored scroll view to the newly added one.

However, there are a few properties for which a WKWebView.scrollView
(WKScrollView, a private class) and UIScrollView have different
defaults. This means that the property copying phase overwrites the
WKWebView.scrollView defaults with the UIScrollView defaults.

A better solution might be to track which properties are set, and only
copy over those properties in the copying phase, but that would be a
much more involved solution.

Bug: 1156224
Change-Id: Ie0ad8bad0439caf3fdb4ec37d03369c43d0a7100
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580533Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836073}

ios/web/web_state/ui/crw_web_view_scroll_view_proxy.mm

@@ -169,6 +169,15 @@ static int gAnyContext = 0;
     // -asUIScrollView method.
     _underlyingScrollView = [[UIScrollView alloc] init];
 
+    // There are a few properties where the default WKWebView.scrollView has
+    // different values from a base UIScrollView. As _underlyingScrollView
+    // starts out as a base UIScrollView, the property preservation code will
+    // copy over these incorrect values and overwrite the default
+    // WKWebView.scrollView values for those properties. Instead, set those
+    // values to their WebKit defaults.
+    _underlyingScrollView.alwaysBounceVertical = YES;
+    _underlyingScrollView.directionalLockEnabled = YES;
+
     [self.class startObservingScrollView:_underlyingScrollView proxy:self];
   }
   return self;