Block an invalid UTF-16 string from findPlainText

We should not pass an invalid UTF-16 input to ICU's
usearch_setPattern(). Blocking it on the ICU's end does not
help in terms of the efficiency (the same check has to be done but only
further down the code path.)

BUG=360431
TEST=1. webkit_unit_tests --gtest_filter=*findPlainTextInval*
2. Do the above with an ASAN build and it should not crash.

Review URL: https://codereview.chromium.org/229193002

git-svn-id: svn://svn.chromium.org/blink/trunk@171298 bbb929c8-8fbe-4397-9dbb-9b2b20218538