Changes window size max for Dictionary-Compressed Zstandard

The spec of Compression Dictionary Transport has changed to allow larger
window size for Dictionary-Compressed Zstandard.
httpwg/http-extensions#2754
So this CL changes the window_log_max in ZstdSourceStream to follow the
spec change.

Before this CL:
  window size = max(8MB, dictionary size)

After this CL:
  window size = clamp(dictionary size * 1.25, 8MB, 128MB)

Bug: 1413922, 1479809
Change-Id: I28853afd8e61107c3d1df3a3eb3f72d99b6fa796
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5561364
Reviewed-by: Adam Rice <[email protected]>
Commit-Queue: Tsuyoshi Horo <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1304911}

net/filter/zstd_source_stream.cc

@@ -47,13 +47,18 @@ class ZstdSourceStream : public FilterSourceStream {
     // to '... protect decoders from unreasonable memory requirements'.
     int window_log_max = 23;
     if (dictionary_) {
-      // For shared dictionary case, allow using larger window size (Log2Ceiling
-      // of `dictionary_size`). It is safe because we have the size limit per
-      // shared dictionary and the total dictionary size limit.
-      window_log_max =
-          std::max(base::bits::Log2Ceiling(
-                       base::checked_cast<uint32_t>(dictionary_size_)),
-                   window_log_max);
+      // For shared dictionary case, allow using larger window size:
+      //   clamp(dictionary size * 1.25, 8MB, 128MB)
+      // See https://github.com/httpwg/http-extensions/issues/2754 for more
+      // details. To avoid floating point calculations, using `* 5 / 4` for
+      // `* 1.25` specified by the standard.
+      // Note: `base::checked_cast<uint32_t>` is safe because we have the size
+      // limit per shared dictionary and the total dictionary size limit.
+      window_log_max = std::clamp(
+          base::bits::Log2Ceiling(
+              base::checked_cast<uint32_t>(dictionary_size_ * 5 / 4)),
+          23,   // 8MB
+          27);  // 128MB
     }
     ZSTD_DCtx_setParameter(dctx_.get(), ZSTD_d_windowLogMax, window_log_max);
     if (dictionary_) {