Revert "Support a prebuilt WebView."

To work around framework resource IDs not being constant, go back to
building webviewchromium.jar from source.

This reverts commit 9c9341f1d2368b4209fe982f3f1c5de384f67fb7.

Bug: 11426593
Merge remote-tracking branch 'cm/android-4.4' into android-4.4