Make WebViewImpl::setCompositionFromExistingText() to work with textarea

This patch makes WebViewImpl::setCompositionFromExistingText() to work with textarea by changing InputMethodController::setCompositionFromExistingText() to make editable DOM tree to single text node. Because InputMethodController doesn't support composition spanning on multiple nodes. Composition can cover multiple nodes by setCompositionFromExistingText() where start offset and end offset are calculated based on character position in result of plaintext() for root editable node of selection.

This patch fixes multiple node spanning composition for textarea only. Issue 276776 will fixes others, e.g. <div contenteditable>foo <span style="display:none">bar</span> baz</div>, and removes HTMLTextFormControlElement::simplifyInnerText().

BUG=275457
TEST=webkit_unit_tests WebViewTest.SetCompositionFromExistingTextInTextArea

Review URL: https://chromiumcodereview.appspot.com/22900013

git-svn-id: svn://svn.chromium.org/blink/trunk@156473 bbb929c8-8fbe-4397-9dbb-9b2b20218538