Add WebView Safe Browsing error code

This adds a Safe Browsing specific network error code for WebView. This
will be returned when we cancel page load because the end user has
clicked "Back to safety" on an interstitial page, the WebView is too
small to display a readable interstitial page, or other similar cases.

When this is used due to an unsafe subresource, the error passed to
WebViewClient#OnReceivedError() will be associated with that specific
resource, not the main frame.

Bug: 36007752
Test: N/A
Change-Id: I8b149a9d6e4ed113de0dd8ee051a2934af477128