Symbols should be fetched only for frames in the PHC stacks

[gabrielesvelto]

Symbols should be downloaded based on which modules appear on the PHC stacks. Right now symbols are being downloaded based on which modules appear in the raw crash report with direct links to the symbol server. That represents only the modules that had symbols fetched because they were present in the crash stack. The PHC stacks might use those modules, might not use them or might use different ones. This currently leaves unsymbolicated frames unless the PHC stacks hit exactly the same modules as the crash stack.

Fortunately it's easy to generate the proper URLs to fetch the symbols dynamically as every module entry contains the module name and code ID which are the only bits required for the URL. See here for how to build the URL.