Expose WebView#onCheckIsTextEditor

WebView was never returning correct return value in
onCheckIsTextEditor(). However, startinput optimization work at
ag/3455281, requires a correct return value when switching between
windows. Chrome-side change (crrev.com/c/952217) has dropped into
66.0.3359.14.

BUG: 74199870
Test: manually verified
Change-Id: I954365a70f1ff2680ed9a6d76e695c1193a4cfa3