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In the current code base, a float number is used when users try to control
the volume slider by using keyboard. It increases/decreases the volume by
0.1 (or 10%) after each press. In the CL2281173002, it accidentally
changed this increment to an integer, so the volume either jumps to silent
or max.

BUG=631775

Review-Url: https://codereview.chromium.org/2297833002
Cr-Commit-Position: refs/heads/master@{#415700}