Move some sniffer arrays to read-only data segment

Due to a quirk in VC++ it is easy to accidentally prevent a const global
array from being placed in the read-only data segment. This change
removes some should-be-harmless 'const' keywords to work around this
quirk and move ~2,500 bytes to the .rdata (read-only) data segment
across chrome.dll and chrome_child.dll. The arrays affected include
kMagicNumbers, kExtraMagicNumbers, kSniffableTags, kOfficeExtensionTypes,
and a few others.

VC++ bug is filed here:
https://connect.microsoft.com/VisualStudio/feedback/details/3117602

Other instances of this quirk are also being worked around.

BUG=677351

Review-Url: https://codereview.chromium.org/2607993002
Cr-Commit-Position: refs/heads/master@{#441007}

net/base/mime_sniffer.cc

@@ -108,11 +108,11 @@ namespace net {
 static const size_t kBytesRequiredForMagic = 42;
 
 struct MagicNumber {
-  const char* const mime_type;
-  const char* const magic;
+  const char* mime_type;
+  const char* magic;
   size_t magic_len;
   bool is_string;
-  const char* const mask;  // if set, must have same length as |magic|
+  const char* mask;  // if set, must have same length as |magic|
 };
 
 #define MAGIC_NUMBER(mime_type, magic) \
@@ -203,7 +203,7 @@ enum OfficeDocType {
 
 struct OfficeExtensionType {
   OfficeDocType doc_type;
-  const char* const extension;
+  const char* extension;
   size_t extension_len;
 };