[wasm] Flip section code for exception and data-count.

This flips the section codes for the exception section (part of the
exception handling proposal) and the data-count section (part of the
bulk memory proposal).

Also see the following discussions for the underlying motivation:
https://github.com/WebAssembly/bulk-memory-operations/pull/42
https://github.com/WebAssembly/exception-handling/issues/70

R=binji@chromium.org
BUG=v8:8868

Change-Id: I9f5f524e7aec067720346a5bb8d45c7c80a4c7a2
Reviewed-on: https://chromium-review.googlesource.com/c/1478207Reviewed-by: Ben Smith <binji@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59856}

src/wasm/wasm-constants.h

@@ -71,8 +71,8 @@ enum SectionCode : int8_t {
   kElementSectionCode = 9,     // Elements section
   kCodeSectionCode = 10,       // Function code
   kDataSectionCode = 11,       // Data segments
-  kExceptionSectionCode = 12,  // Exception section
-  kDataCountSectionCode = 13,  // Number of data segments
+  kDataCountSectionCode = 12,  // Number of data segments
+  kExceptionSectionCode = 13,  // Exception section
 
   // The following sections are custom sections, and are identified using a
   // string rather than an integer. Their enumeration values are not guaranteed
@@ -83,7 +83,7 @@ enum SectionCode : int8_t {
   // Helper values
   kFirstSectionInModule = kTypeSectionCode,
   kLastKnownModuleSection = kSourceMappingURLSectionCode,
-  kFirstUnorderedSection = kExceptionSectionCode,
+  kFirstUnorderedSection = kDataCountSectionCode,
 };
 
 // Binary encoding of name section kinds.

test/mjsunit/wasm/wasm-module-builder.js

@@ -70,8 +70,8 @@ let kStartSectionCode = 8;       // Start function declaration
 let kElementSectionCode = 9;     // Elements section
 let kCodeSectionCode = 10;       // Function code
 let kDataSectionCode = 11;       // Data segments
-let kExceptionSectionCode = 12;  // Exception section (between Global & Export)
-let kDataCountSectionCode = 13;  // Data segments
+let kDataCountSectionCode = 12;  // Data segment count (between Element & Code)
+let kExceptionSectionCode = 13;  // Exception section (between Global & Export)
 
 // Name section types
 let kModuleNameCode = 0;