Update layout at required place in InputMethodController::setComposition()

This patch moves update layout call before |mostForwardCaretPosition()|
which requires clean layout tree as a preparation of introducing clean layout
tree assertion to |mostForwardCaretPosition()|.

BUG=590369
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2147303003
Cr-Commit-Position: refs/heads/master@{#406246}
1 file changed