Clean up -Wbitfield-conversion warnings

Clang has recently introduced a new warning -Wbitfield-conversion which
warns when a conversion from an integral type to a bit-field may change
the value. To suppress this warning, we use a bitmask to ensure that the
value is within the specified range.

Change-Id: Ie771475e17e37d73256532decde15445752a0e3d

include/perfetto/ext/tracing/core/shared_memory_abi.h

@@ -424,7 +424,7 @@ class SharedMemoryABI {
       ChunkHeader* chunk_header = header();
       auto packets = chunk_header->packets.load(std::memory_order_relaxed);
       if (packets.count < packet_count)
-        packets.count = packet_count;
+        packets.count = packet_count & ChunkHeader::Packets::kMaxCount;
       chunk_header->packets.store(packets, std::memory_order_release);
       return packets.count;
     }

include/perfetto/protozero/field.h

@@ -156,7 +156,7 @@ class Field {
                   uint8_t type,
                   uint64_t int_value,
                   uint32_t size) {
-    id_ = id;
+    id_ = id & kMaxId;
     type_ = type;
     int_value_ = int_value;
     size_ = size;

src/tracing/core/trace_buffer.cc

@@ -135,7 +135,7 @@ void TraceBuffer::CopyChunkUntrusted(ProducerID producer_id_trusted,
   record.chunk_id = chunk_id;
   record.writer_id = writer_id;
   record.num_fragments = num_fragments;
-  record.flags = chunk_flags;
+  record.flags = chunk_flags & ChunkRecord::kFlagsBitMask;
   ChunkMeta::Key key(record);
 
   // Check whether we have already copied the same chunk previously. This may
@@ -452,7 +452,7 @@ bool TraceBuffer::TryPatchChunkContents(ProducerID producer_id,
   stats_.set_patches_succeeded(stats_.patches_succeeded() + patches_size);
   if (!other_patches_pending) {
     chunk_meta.flags &= ~kChunkNeedsPatching;
-    chunk_record->flags = chunk_meta.flags;
+    chunk_record->flags = chunk_meta.flags & ChunkRecord::kFlagsBitMask;
   }
   return true;
 }

src/tracing/core/trace_buffer.h

@@ -339,6 +339,8 @@ class TraceBuffer {
     uint16_t size;
 
     uint8_t flags : 6;  // See SharedMemoryABI::ChunkHeader::flags.
+    static constexpr size_t kFlagsBitMask = (1 << 6) - 1;
+
     uint8_t is_padding : 1;
     uint8_t unused_flag : 1;