Signed exchange: Increase the header size limit to 512kB

Spec side change: https://github.com/WICG/webpackage/pull/261

Bug: 803774
Change-Id: I0d0aa289b6f991b3cb09b2f53f6b1fbf752d2bbd
Reviewed-on: https://chromium-review.googlesource.com/1142846Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576371}

content/browser/web_package/signed_exchange_prologue.cc

@@ -14,7 +14,7 @@ namespace {
 
 constexpr char kSignedExchangeMagic[] = "sxg1-b1";
 constexpr size_t kMaximumSignatureHeaderFieldLength = 16 * 1024;
-constexpr size_t kMaximumCBORHeaderLength = 16 * 1024;
+constexpr size_t kMaximumCBORHeaderLength = 512 * 1024;
 
 }  // namespace