media/gpu/vaapi: increase the upper bound of QP.

QP value is used to control the quality and the bitrate of encoded
video stream. When QP value increases, the bitrate will decreased.

Originally the bitrate is higher than expected value when encoding
small resolution video. This CL increases the upper bound of QP value,
let the encoder could encode the video with lower bitrate.

BUG=b:110059922
BUG=chromium:974593
TEST=Run video_encode_accelerator_unittest on Eve and Cyan

Change-Id: I4922989f279052d2d19a87b1b858554c51120249
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660376Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669587}

media/gpu/vaapi/vp8_encoder.cc

@@ -18,7 +18,8 @@ constexpr int kCPBWindowSizeMs = 1500;
 
 // Based on WebRTC's defaults.
 constexpr int kMinQP = 4;
-constexpr int kMaxQP = 112;
+// b/110059922: Tuned 112->113 for bitrate issue in a lower resolution (180p).
+constexpr int kMaxQP = 113;
 constexpr int kDefaultQP = (3 * kMinQP + kMaxQP) / 4;
 }  // namespace