Views Textfield: copy-on-select and paste-on-middle-click.

Add Textfield::PasteSelectionClipboard helper function.
(used to paste on middle click, update cursor/selection)
Add file-local UpdateSelectionClipboard helper function.
(used to write non-empty text to the selection clipboard)

Call UpdateAfterChange consistently after changes.
(not just OnCaretBoundsChanged, SchedulePaint, etc.)
Cleanup ExecuteCommand, OnMousePressed/Dragged, etc.

Add TextfieldTest.SelectionClipboard unit tests.
Make Get/SetClipboardText helpers file-local.

BUG=319011
R=erg@chromium.org,oshima@chromium.org
TEST=Copy-on-select and paste-on-middle-click work for Linux Aura omnibox and browser textfields (find-in-page, bookmark bubble).
NOTRY=true

Review URL: https://codereview.chromium.org/73403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246478 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246478