Remove call to rebuildWebTextView from handling selection change.

The call was originally added to fix bug 2431351, where touching
a <textarea> sent a message with the new selection, but the UI
thread had not yet updated the WebTextView, so the new selection
was ignored.  However, since then,
https://android-git.corp.google.com/g/#change,45476 was submitted
to pass the selection and open the keyboard at the same time. So
when UPDATE_TEXT_SELECTION_MSG_ID is received, we no longer need
to worry about the case where the WebTextView is not set up yet.

Bug 2600847

Change-Id: I19eb102a7188e6edafaa5ac17b19ebd4ab5764f0