Changes StackTraceDeobfuscator to abstract and provide various factory methods.

This change will remove the need for extending StackTraceObfusctor most of the
time as in almost all scenarios it is extended to get the symbol map from jar
through the class loader (which makes sense).
Also a factory is provided to get the symbol map from the file system. For all
other cases, developers can construct the deobfuscator with a URL path or extend
the class to override openInputStream method.

Change-Id: I562e052caef8da7f3434319cf11b8984bc347fe5
Review-Link: https://gwt-review.googlesource.com/#/c/2270/

Review by: skybrian@google.com