Don't define log2 and log2f in blink for Android

The definition of log2f in blink was causing problems on Windows and
is no longer necessary. It is also unnecessary on Android and may
also be causing problems there, so this change finishes removing
the definitions.

See crrev.com/2556383002 for details.

Review-Url: https://codereview.chromium.org/2556383002
Cr-Commit-Position: refs/heads/master@{#437351}
1 file changed