TraceProcessor: increase HTTP max req size

Unlike previously assumed, the UI url-base fetcher doesn't
guarantee that chunks are < 32MB, it only guarantees to tokenize
on the first chunk > 32MB.
The 32MB limit on httpd ended up to be a bit too strict. Just
relax it.

Test: manual on Mac and Windows:
  1. start trace_processor_shell -D,
  2. Open ui.perfetto.dev (RPC will light green)
  3. Open the example android trace.

Change-Id: Ia93b9398d091a12aae56814fda5357a48b84d0f8
Bug: 209578860
diff --git a/src/base/http/http_server.cc b/src/base/http/http_server.cc
index 17a1652..ebb57e7 100644
--- a/src/base/http/http_server.cc
+++ b/src/base/http/http_server.cc
@@ -29,7 +29,7 @@
 namespace base {
 
 namespace {
-constexpr size_t kMaxPayloadSize = 32 * 1024 * 1024;
+constexpr size_t kMaxPayloadSize = 64 * 1024 * 1024;
 constexpr size_t kMaxRequestSize = kMaxPayloadSize + 4096;
 
 enum WebsocketOpcode : uint8_t {