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Before, when entering contracted (gradd 2) braille, backtracking was done
using the chrome.input.ime.deleteSurroundingText API.  This made text entry
visible in the text field immediately.  Unfortunately, some contexts don't
handle programmatic deletion well (Google Docs' editor is an example).

The new approach is to not commit the text ot the edit field until a word
has been entered or a non-braille key is entered. To make the user experience
better for the braille user, entered braille cells are shown on the
display right before the curent selection so that the user gets immediate
feedback when typing on the braille dipslay.

This approach has drawbacks as well. For example, clicking with a mouse
while typing could move focus before the IME has a chance to submit the
in-progress word. The assumption is that this would be less of a
problem in practice then not being able to use grade 2 at all in some
situations.

BUG=392854

Review URL: https://codereview.chromium.org/1039703002

Cr-Commit-Position: refs/heads/master@{#330711}