[ubsan] Fix double->float cast in blink::ToFloat

Casting is undefined behavior if the value can't be represented
in the target type.

Bug: chromium:932472
Change-Id: I44d2b95512e25f1be94e3d78b9c86e212ab103c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497386
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638887}
1 file changed