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Before this patch, Blink clears selection when focus is changed. This patch
changes to clear selection for text field only. Thus, we keep selection for
content editable.

This patch also updates below tests to follow new behavior introduced by this
patch.

- fast/dom/blur-contenteditable.html
 Render tree dump has a caret.
- fast/layers/scroll-rect-to-visible.html
 Render tree dump has a caret.
- fast/events/selectionchange-user-initiated.html
 One "selectionchange" event from clear selection for content editable has
 been gone.
- /fast/events/selectstart-prevent-selectall.html
 same as above.
- fast/forms/focus-selection-input.html
 One "selectionchange" event from clear selection for caret on LABEL element
 by test step 5 been gone.
- fast/forms/focus-selection-textarea.html
 same as above

BUG=351981
TEST=LayoutTests/editing/selection/keep-selection-after-set-focus.html

Review URL: https://codereview.chromium.org/424973003

git-svn-id: svn://svn.chromium.org/blink/trunk@180228 bbb929c8-8fbe-4397-9dbb-9b2b20218538