Add a test to verify that enabling ImeThread can avoid crash after paste

Chrome can crash if pasting long text into textarea, because there is an
overflow bug in SpannableStringBuilder#replace(). If ImeThread is
enabled, we don't need to call the above function. Thus, we can get rid
of that bug.

BUG=606059
Committed: https://crrev.com/bd10c315594d2a20e31a94a7a6c7adb9a0961c56
Cr-Commit-Position: refs/heads/master@{#399850}

[yabinh, 2016-06-03 12:46:29]

Description was changed from

==========
Trybot test for crash after paste
==========

to

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow bug in SpannableStringBuilder#replace(). If ImeThread is enabled, we
don't need to call the above function. Thus, we can get rid of that bug.

BUG=606059
==========

[yabinh, 2016-06-03 12:46:30]

yabinh@chromium.org changed reviewers:
+ aelias@chromium.org, changwan@chromium.org

[yabinh, 2016-06-03 14:41:20]

Description was changed from

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow bug in SpannableStringBuilder#replace(). If ImeThread is enabled, we
don't need to call the above function. Thus, we can get rid of that bug.

BUG=606059
==========

to

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow     
bug in SpannableStringBuilder#replace(). If ImeThread is enabled, we don't need 
  
to call the above function. Thus, we can get rid of that bug.

BUG=606059
==========

[yabinh, 2016-06-03 14:46:15]

Description was changed from

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow     
bug in SpannableStringBuilder#replace(). If ImeThread is enabled, we don't need 
  
to call the above function. Thus, we can get rid of that bug.

BUG=606059
==========

to

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow bug in SpannableStringBuilder#replace(). If ImeThread is 
enabled, we don't need to call the above function. Thus, we can get rid 
of that bug.

BUG=606059
==========

[aelias_OOO_until_Jul13, 2016-06-03 19:43:41]

lgtm

[Changwan Ryu, 2016-06-07 00:31:24]

lgtm

https://codereview.chromium.org/2033723003/diff/20001/content/public/android/...
File
content/public/android/javatests/src/org/chromium/content/browser/input/ImeTest.java
(right):

https://codereview.chromium.org/2033723003/diff/20001/content/public/android/...
content/public/android/javatests/src/org/chromium/content/browser/input/ImeTest.java:633:
int textLength = 25000;
nit: according to stackoverflow, this is the shortest version:

String text = new String(new char[25000]).replace("\0", "a");

[yabinh, 2016-06-15 05:09:44]

The CQ bit was checked by yabinh@chromium.org

[yabinh, 2016-06-15 05:09:44]

The patchset sent to the CQ was uploaded after l-g-t-m from aelias@chromium.org,
changwan@chromium.org
Link to the patchset: https://codereview.chromium.org/2033723003/#ps30001
(title: "a small change for changwan's review")

[commit-bot: I haz the power, 2016-06-15 05:09:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2033723003/30001

[commit-bot: I haz the power, 2016-06-15 05:40:41]

Description was changed from

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow bug in SpannableStringBuilder#replace(). If ImeThread is 
enabled, we don't need to call the above function. Thus, we can get rid 
of that bug.

BUG=606059
==========

to

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow bug in SpannableStringBuilder#replace(). If ImeThread is 
enabled, we don't need to call the above function. Thus, we can get rid 
of that bug.

BUG=606059
==========

[commit-bot: I haz the power, 2016-06-15 05:40:42]

Committed patchset #3 (id:30001)

[commit-bot: I haz the power, 2016-06-15 05:40:47]

CQ bit was unchecked

[commit-bot: I haz the power, 2016-06-15 05:43:46]

Description was changed from

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow bug in SpannableStringBuilder#replace(). If ImeThread is 
enabled, we don't need to call the above function. Thus, we can get rid 
of that bug.

BUG=606059
==========

to

==========
Chrome can crash if pasting long text into textarea, because there is an
overflow bug in SpannableStringBuilder#replace(). If ImeThread is
enabled, we don't need to call the above function. Thus, we can get rid
of that bug.

BUG=606059

Committed: https://crrev.com/bd10c315594d2a20e31a94a7a6c7adb9a0961c56
Cr-Commit-Position: refs/heads/master@{#399850}
==========

[commit-bot: I haz the power, 2016-06-15 05:43:47]

Patchset 3 (id:??) landed as
https://crrev.com/bd10c315594d2a20e31a94a7a6c7adb9a0961c56
Cr-Commit-Position: refs/heads/master@{#399850}