Fix for building with ancient toolchain

At Opera we test our SDK on an old MIPS device with a
peculiar toolchain. To be able to compile ImageDocument.o,
we need to call std::round(), not just round().

BUG=none

Review-Url: https://codereview.chromium.org/2548783002
Cr-Commit-Position: refs/heads/master@{#440950}
1 file changed