Enable debugging symbols to always use DWARF-4 format.

Adding -gdwarf-4 to make DWARF-4 the output format.
 * gcc 4.6 outputs DWARF-2 by default
 * gcc 4.7 outputs DWARF-4 by default
 * clang <3.4 outputs DWARF-3 by default
 * clang >3.4 outputs DWARF-4 by default

DWARF-4 is the latest debug output format which includes features for
macro-expansion, better var tracking and other similar options.

From http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html
> -gdwarf-4
>  Produce debugging information in DWARF format (if that is
>  supported). The value of version may be either 2, 3 or 4; the
>  default version for most targets is 4.  Note that with DWARF
>  Version 2, some ports require and always use some non-conflicting
>  DWARF 3 extensions in the unwind tables.
>
>  Version 4 may require GDB 7.0 and -fvar-tracking-assignments for
>  maximum benefit.

BUG=

Review URL: https://codereview.chromium.org/198363002

Cr-Commit-Position: refs/heads/master@{#295933}

build/common.gypi

@@ -3558,6 +3558,7 @@
             'cflags': [
               '-O>(debug_optimize)',
               '-g',
+              '-gdwarf-4',
             ],
             'conditions' : [
               ['OS=="android"', {