WebView doc: explain persist js across navigations
WebView doc: explain persist js across navigations

BUG: 28207652
Change-Id: Ib33f029c2cae6c1482c0c951075275e9f96ae73c

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base