Bug 563195:  Avoid triggering -Wshadow warnings in mfbt/.  r=Waldo
Bug 563195:  Fix -Wshadow warnings in public JS API headers (those used from layout/style/).  r=njn
Bug 563195:  Fix -Wshadow warnings in layout/style.  r=bzbarsky
Bug 563195:  Fix -Wshadow warnings in public SPS headers.  r=BenWa
Bug 563195:  Fix -Wshadow warnings in gfx that affect layout/style.  r=derf

A better name here would be welcome.
Bug 563195:  Add -Wshadow to CFLAGS/CXXFLAGS for layout/style.  r=ted

Since this directory has FAIL_ON_WARNINGS set, this will cause any
shadowing warnings triggered by headers included in layout/style to
cause errors.

Once this patch is reviewed, I'll write and attach patches to fix the
warnings needed to land this.