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Both the web and ARC++ IME frameworks support moving the caret after
InsertText, but this functionality is not exposed to IMEs. Our new IME
needs this feature, so this patch adds an option to move the cursor
to TextInputClient.

Unfortunately, the web and ARC++ APIs are slightly different:
- Web allows the the cursor to be moved anywhere.
- ARC++ doesn't allow the cursor to be moved inside the inserted text.

Therefore, although both web and ARC++ take in an integer cursor position
parameter, they have very different meanings. To avoid confusion, we use an
enum to indicate whether the cursor should be before or after the inserted
text (since that's the minimal feature that the IME needs).

To keep this CL small, we will first introduce the enum with only a single value
(move cursor after text). A future CL will add the additional parameter.

Bug: 1155331
Change-Id: I5eb9e5c23fef290b9962b1dee327c76343f08f5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2576062Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835087}