Refactor some WebView loading logic into WebViewLibraryLoader.

This is in preparation for a patch which with bigger changes for
WebViewLibraryLoader (including tests).

Bug: 28736099
Test: ensure relro loaded into app process on 32-bit device.
Change-Id: Ic1aef697c1010380d8bce466ee14627d9cdff9e7
2 files changed