Fix clank compilation errors in the relocation_packer.

Fix clank compilation errors in the relocation_packer.

Add nugatory constructor and destructor for Leb128Encoder.  Add nugatory
destructor for Leb128Decoder.  Move constructor for Leb128Decoder from
leb128.h into leb128.c and un-simplify.

Fixes:
  tools/relocation_packer/src/leb128.h:23:1: error: [chromium-style] Complex class/struct needs an explicit out-of-line constructor.
  tools/relocation_packer/src/leb128.h:23:1: error: [chromium-style] Complex class/struct needs an explicit out-of-line destructor.
  tools/relocation_packer/src/leb128.h:47:3: error: [chromium-style] Complex constructor has an inlined body.
  tools/relocation_packer/src/leb128.h:43:1: error: [chromium-style] Complex class/struct needs an explicit out-of-line destructor.

BUG=385553

Review URL: https://codereview.chromium.org/334653008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@280388 0039d316-1c4b-4281-b951-d872f2087c98