Call libyuv for short->half-float conversion

Libyuv has a more optimized version of the half-float conversion code now, and it
will probably become more optimized in the future.

BUG=445071
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2389003002
Cr-Commit-Position: refs/heads/master@{#422659}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 52a364a..524c485 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -583,6 +583,7 @@
     "//gpu",
     "//gpu/command_buffer/client:gles2_interface",
     "//media",
+    "//third_party/libyuv",
     "//ui/events:events_base",
     "//ui/gfx",
     "//ui/gfx/geometry",