Calculate memory thresholds that V8 can use to control GC.

V8 can be smarter about the memory it uses and the frequency
of garbage collection but we need to inform it of the values
to use. As this varies device to device look it up and store
it on the WebViewCore to be read by native code over JNI.

See also https://android-git.corp.google.com/g/#change,78691

Bug: 3075565

Change-Id: Ib52f4df775b80b386fc98f1d71b5687f01c12b2e