Refactor FontResource to avoid SharedBuffer::Data()

Since PackageFormatOf() only needs the first 4 bytes, just copy them to
a local buffer.

BUG=728627

Change-Id: I19e65c95f4737968967fb63196b8ed6d07fa7fae
Reviewed-on: https://chromium-review.googlesource.com/577673Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488201}

third_party/WebKit/Source/core/loader/resource/FontResource.cpp

@@ -55,10 +55,11 @@ enum FontPackageFormat {
 };
 
 static FontPackageFormat PackageFormatOf(SharedBuffer* buffer) {
-  if (buffer->size() < 4)
+  static constexpr size_t kMaxHeaderSize = 4;
+  char data[kMaxHeaderSize];
+  if (!buffer->GetBytes(data, kMaxHeaderSize))
     return kPackageFormatUnknown;
 
-  const char* data = buffer->Data();
   if (data[0] == 'w' && data[1] == 'O' && data[2] == 'F' && data[3] == 'F')
     return kPackageFormatWOFF;
   if (data[0] == 'w' && data[1] == 'O' && data[2] == 'F' && data[3] == '2')