[wasm] Add support and unittests for decoding signed LEB128.

test/unittests/unittests.gyp

 # Copyright 2014 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'v8_code': 1,
   },
   'includes': ['../../build/toolchain.gypi', '../../build/features.gypi'],
   'targets': [
@@ skipping 100 matching lines @@
         'heap/memory-reducer-unittest.cc',
         'heap/heap-unittest.cc',
         'heap/scavenge-job-unittest.cc',
         'heap/slot-set-unittest.cc',
         'heap/remembered-set-unittest.cc',
         'locked-queue-unittest.cc',
         'run-all-unittests.cc',
         'test-utils.h',
         'test-utils.cc',
         'wasm/ast-decoder-unittest.cc',
+        'wasm/decoder-unittest.cc',
         'wasm/encoder-unittest.cc',
         'wasm/loop-assignment-analysis-unittest.cc',
         'wasm/module-decoder-unittest.cc',
         'wasm/wasm-macro-gen-unittest.cc',
       ],
       'conditions': [
         ['v8_target_arch=="arm"', {
           'sources': [  ### gcmole(arch:arm) ###
             'compiler/arm/instruction-selector-arm-unittest.cc',
           ],
@@ skipping 66 matching lines @@
             '../../build/isolate.gypi',
           ],
           'sources': [
             'unittests.isolate',
           ],
         },
       ],
     }],
   ],
 }