Revert "Move some sniffer arrays to read-only data segment"

This reverts commit 40b53de7 now that we
don't need to work around the VC++ bug.

Bug: 934323
Change-Id: I7075d80778737666a906007b73b4a53d350fa772
Reviewed-on: https://chromium-review.googlesource.com/c/1484413
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634882}

net/base/mime_sniffer.cc

@@ -101,11 +101,11 @@ namespace net {
 static const size_t kBytesRequiredForMagic = 42;
 
 struct MagicNumber {
-  const char* mime_type;
-  const char* magic;
+  const char* const mime_type;
+  const char* const magic;
   size_t magic_len;
   bool is_string;
-  const char* mask;  // if set, must have same length as |magic|
+  const char* const mask;  // if set, must have same length as |magic|
 };
 
 #define MAGIC_NUMBER(mime_type, magic) \
@@ -196,7 +196,7 @@ enum OfficeDocType {
 
 struct OfficeExtensionType {
   OfficeDocType doc_type;
-  const char* extension;
+  const char* const extension;
   size_t extension_len;
 };