Add monitoring function for preedit text of input method.

This is going to be used for fixing crosbug.com/13374, by
adding a display area for preedits on a candidate window.

BUG=chromium-os:13374
TEST=boot ChromeOS and verify it runs properly.

Change-Id: Ib3c467a3e0df235f920225894addee7a0a78a058
Reviewed-on: http://gerrit.chromium.org/gerrit/2288
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Tested-by: Kazuhiro Inaba <kinaba@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/2436
Reviewed-by: Kazuhiro Inaba <kinaba@chromium.org>
Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
Tested-by: Takayoshi Kochi <kochi@chromium.org>

• chromeos_cros_api.h[diff]
• chromeos_input_method_ui.cc[diff]
• chromeos_input_method_ui.h[diff]
• load.cc[diff]