Fixed AppsGridViewTest failures with experimental app list enabled.
Now if IsExperimentalAppListEnabled returns true, the tests still pass.
1. Fixed AppsGridViewTest::GetItemTileRectAt to take padding into
account.
2. MouseDragMaxItemsInFolderWithMovement drags to a point inside the
tile rect, not 1px outside.
3. Double size of test grid view (from 320x240 to 640x480).
MouseDragItemReorder was failing due to increased size of the tiles
(test drop targets were outside the window bounds).
BUG=437436
Review URL: https://codereview.chromium.org/
768713003
Cr-Commit-Position: refs/heads/master@{#306148}