vaapi/vp8_encoder: optimize with constexpr usage

Replace some of the const defintions with constexpr
as a part of optimization based on review comments
for https://chromium-review.googlesource.com/c/chromium/src/+/1383540

Change-Id: I82e8955950f526a2beb52f281efb6b9a463f1761
Reviewed-on: https://chromium-review.googlesource.com/c/1457423Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629886}

media/gpu/vaapi/vp8_encoder.cc

@@ -11,15 +11,15 @@ namespace media {
 
 namespace {
 // Keyframe period.
-const size_t kKFPeriod = 3000;
+constexpr size_t kKFPeriod = 3000;
 
 // Arbitrarily chosen bitrate window size for rate control, in ms.
-const int kCPBWindowSizeMs = 1500;
+constexpr int kCPBWindowSizeMs = 1500;
 
 // Based on WebRTC's defaults.
-const int kMinQP = 4;
-const int kMaxQP = 112;
-const int kDefaultQP = (3 * kMinQP + kMaxQP) / 4;
+constexpr int kMinQP = 4;
+constexpr int kMaxQP = 112;
+constexpr int kDefaultQP = (3 * kMinQP + kMaxQP) / 4;
 }  // namespace
 
 VP8Encoder::EncodeParams::EncodeParams()