Revert "[wasm] Gracefully handle malformed custom sections in WebAssembly.Module.customSections()."

This reverts commit 163c1c82.

Reason for revert: Throws std::bad_alloc on linux: https://build.chromium.org/p/client.v8/builders/V8%20Linux/builds/21927; needs investigation.

Original change's description:
> [wasm] Gracefully handle malformed custom sections in WebAssembly.Module.customSections().
> 
> R=​clemensh@chromium.org
> BUG=chromium:789952
> 
> Change-Id: Ida627fa6cdeacff01a0ec4d20e58281f17528010
> Reviewed-on: https://chromium-review.googlesource.com/800941
> Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
> Commit-Queue: Ben Titzer <titzer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49767}

TBR=titzer@chromium.org,clemensh@chromium.org

Change-Id: I63fbd8f55025f53c453e91d0f7a181c21ae53a39
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:789952
Reviewed-on: https://chromium-review.googlesource.com/801554Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49768}

src/wasm/module-decoder.cc

@@ -1544,13 +1544,8 @@ std::vector<CustomSectionOffset> DecodeCustomSections(const byte* start,
     uint32_t name_offset = decoder.pc_offset();
     decoder.consume_bytes(name_length, "section name");
     uint32_t payload_offset = decoder.pc_offset();
-    if (section_length < (payload_offset - section_start)) {
-      decoder.error("invalid section length");
-      break;
-    }
     uint32_t payload_length = section_length - (payload_offset - section_start);
     decoder.consume_bytes(payload_length);
-    if (decoder.failed()) break;
     result.push_back({{section_start, section_length},
                       {name_offset, name_length},
                       {payload_offset, payload_length}});

test/mjsunit/regress/wasm/regress-789952.js

-// Copyright 2017 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.
-
-var string_len = 0x0ffffff0 - 19;
-
-print("Allocating backing store");
-var backing = new ArrayBuffer(string_len + 19);
-
-print("Allocating typed array buffer");
-var buffer = new Uint8Array(backing);
-
-print("Filling...");
-buffer.fill(0x41);
-
-print("Setting up array buffer");
-// Magic
-buffer.set([0x00, 0x61, 0x73, 0x6D], 0);
-// Version
-buffer.set([0x01, 0x00, 0x00, 0x00], 4);
-// kUnknownSection (0)
-buffer.set([0], 8);
-// Section length
-buffer.set([0x80, 0x80, 0x80, 0x80, 0x00],  9);
-// Name length
-buffer.set([0xDE, 0xFF, 0xFF, 0x7F], 14);
-
-print("Parsing module...");
-var m = new WebAssembly.Module(buffer);
-
-print("Triggering!");
-var c = WebAssembly.Module.customSections(m, "A".repeat(string_len + 1));
-assertEquals(0, c.length);