Make testContextMenuOpenInNewTabFromTallPage less flaky.

ios/chrome/browser/context_menu/context_menu_egtest.mm

Index: ios/chrome/browser/context_menu/context_menu_egtest.mm
diff --git a/ios/chrome/browser/context_menu/context_menu_egtest.mm b/ios/chrome/browser/context_menu/context_menu_egtest.mm
index 923a41ed8126b9f97ce4a1d10439a39cdcad8989..a8cfb8038e0ef3baed7ee53a7a15f52678666260 100644
--- a/ios/chrome/browser/context_menu/context_menu_egtest.mm
+++ b/ios/chrome/browser/context_menu/context_menu_egtest.mm
@@ -208,6 +208,12 @@ void SelectTabAtIndexInCurrentMode(NSUInteger index) {
   [ChromeEarlGrey loadURL:initialURL];
   chrome_test_util::AssertMainTabCount(1U);
 
+  // Scroll down on the web view to make sure the toolbar disappears.
+  [[EarlGrey
+      selectElementWithMatcher:WebViewScrollView(
+                                   chrome_test_util::GetCurrentWebState())]
+      performAction:grey_swipeFastInDirection(kGREYDirectionUp)];

[Eugene But (OOO till 7-30), 2017/03/13 22:35:01]

Instead of scrolling twice should we keep scrolling until hit the bottom?

[baxley, 2017/03/13 22:43:41]

This API does (or should) scroll to the bottom. I'll debug a little more. When
it failed, it looked like there was about one toolbar's worth of content to
scroll more to go. I am wondering if the timing of when the toolbar goes away is
messing with the calculation EG is using to find the bottom.

[Eugene But (OOO till 7-30), 2017/03/13 22:54:28]

Maybe this is because WebView's height has changed after the scrolling has
started?

[baxley, 2017/03/14 23:32:10]

Yes, that's my guess. This method is used in fullscreen_egtest (and maybe other
places) to hide the toolbar, maybe I'll make a helper for it. It seems
relatively reliable.

What do you think if I make the following changes?
- add a call to [ChromeEarlGreyUI waitForToolbarVisible:NO] after the first
swipe.
- Create a helper (swipeWebViewToHideToolbar), and then call that in place of
the first swipe. Even if it's calling the same EG method this will make it more
readable, and it will also allow us to come up with a better solution or change
implementation of how we hide the toolbar.

+
   // Scroll down on the web view to make the link visible.
   [[EarlGrey
       selectElementWithMatcher:WebViewScrollView(