Avoid underlining misspellings when typing inside a word

Avoid underlining misspellings when typing inside a word

This CL changes TypingCommand to cancel ongoing spellcheck requests when user
types a new character.

If spellcheck is slow enough to not return after every character that user
types, then spellcheck results can arrive after a user has typed several
characters since spellcheck request. This can happen, for example, if spellcheck
service is accessed over network. If the user is typing in the middle of a word,
then the stale spellcheck result will appear to underline only a part of the
word or include additional space around the word.

TEST=WebFrameTest.SlowSpellcheckMarkerPosition
BUG=179639
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=151707

[please use gerrit instead, 2013-05-31 15:32:28]

Adam: PTAL.

[abarth-chromium, 2013-05-31 17:32:05]

This CL looks reasonable to me, but I'm not sure I'm the right person to review
it.  Is there someone who know more about spell checking who would be a better
reviewer?

[please use gerrit instead, 2013-05-31 17:35:40]

Tony: Do you think you'd be a good reviewer for this CL?

[abarth-chromium, 2013-05-31 17:41:45]

I think Tony is on leave at the moment...

[please use gerrit instead, 2013-05-31 18:07:43]

Hajime: Do you think that you would be a good reviewer for this CL?

[Hajime Morrita, 2013-06-03 00:23:53]

lgtm considering that this fixes a crash.

SpellChecker has a list of pending spellcheck requests. So cancelling only the
last one might not be sufficient.
I'm not sure whether such a queue is worth having. I we need to cancel in-flight
spellcheck per new input, 
the queue should have at most one entry. It seems over-engineering in
retrospect.

[commit-bot: I haz the power, 2013-06-03 17:13:44]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rouslan@chromium.org/16190007/5001

[commit-bot: I haz the power, 2013-06-03 17:24:09]

Retried try job too often on blink_presubmit for step(s) presubmit
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=blink_pres...

[please use gerrit instead, 2013-06-03 17:30:42]

Adam: Need an OWNERS l-g.

[abarth-chromium, 2013-06-03 18:51:53]

test LGTM

[commit-bot: I haz the power, 2013-06-03 18:53:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rouslan@chromium.org/16190007/19001

[commit-bot: I haz the power, 2013-06-04 00:03:53]

Change committed as 151707