Unhide get/setTextClassifier in WebView

We can unhide the methods etTextClassifier and getTextClassifier since
the WebView M61 that is shipping with O-MR1 has the corresponding
implementation.

Bug: 63903071
Test: CTS test in a ceparate CL
Change-Id: I72df0d34a0fccd2c213ed10ca8993a7d786ff835