Support fractional desktopScale values in choosePluginDimensions.

Support fractional desktopScale values in choosePluginDimensions.

Previously desktopScale was applied as part of the scale calculations,
so that snapping to integer scale factors would take effect afterward.
This lead to situations in which the client would be rendered 1:1
host:client device pixels at high-DPI clients, so effectively appearing
down-scaled and often unusably small.

choosePluginDimensions now adjusts the supplied client area dimensions
to account for desktopScale as well as the client's devicePixelRatio.

BUG=498346
Committed: https://crrev.com/7eb7b5d7b22886b30bf1e089ceb346914e5b6d24
Cr-Commit-Position: refs/heads/master@{#338707}

[Wez, 2015-07-14 06:24:57]

wez@chromium.org changed reviewers:
+ sergeyu@chromium.org

[Sergey Ulanov, 2015-07-14 16:18:52]

lgtm

[Wez, 2015-07-14 17:11:57]

The CQ bit was checked by wez@chromium.org

[commit-bot: I haz the power, 2015-07-14 17:12:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1237763002/1

[commit-bot: I haz the power, 2015-07-14 17:23:16]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-14 17:24:03]

Patchset 1 (id:??) landed as
https://crrev.com/7eb7b5d7b22886b30bf1e089ceb346914e5b6d24
Cr-Commit-Position: refs/heads/master@{#338707}