Add optional parameter to WTFReportBacktrace for stackframe count

This patch gives epic yakshavers an optional parameter to control
how many stackframes to show when using WTFReportBacktrace. This
just exposes a previously static framesToShow value.

Users may find this useful in debugging deep traces.

This patch does not modify the BACKTRACE macro because macros
cannot have optional parameters.
This patch does not use templates this function is in a c block which
does not support templates ("templates must have C++ linkage").

TEST=manual

Review URL: https://chromiumcodereview.appspot.com/23494018

git-svn-id: svn://svn.chromium.org/blink/trunk@157400 bbb929c8-8fbe-4397-9dbb-9b2b20218538