[VK] Fix keyboard shortcut documentation.

This patch fixes the ambiguous description in the keyboard shortcut
viewer for Ctrl-Space and Ctrl-Shift-Space. Namely, Ctrl-Space
toggles between the current input method and the last used, whereas
Ctrl-Shift-Space cycles through all of them. The naming is confusing
because "previous" here means "last used" and not the opposite of
"next". The two shortcuts are -not- inverses of each other. We rename
the implementation as well for clarity.

TBR=shuchen@chromium.org

Bug: 909863
Change-Id: I7ca8d70d5d571ce3065814cb789e79a1488b875d
Reviewed-on: https://chromium-review.googlesource.com/c/1358195
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614603}
24 files changed