Specify canvas size by using CSS width/height.

Specify canvas size by using CSS width/height.

In Gallery, the size of the slide image's canvas is scaled by
window.devicePixelRatio. For example, if the devicePixelRatio is 2, the doubled
size canvas is prepared and it is scaled to displaying size so that the pixels
of canvas fit to the device pixels.

Previously the canvas is scaled by using CSS transformation. Thus the element
position is calculated by the original canvas size, and then it is scaled. This
makes the position calculation of canvas complex, and it was regressed at the
crrev.com/398263002.

At this CL, canvas is scaled by using CSS width/height property instead of
bringing back the complex position calculation.

BUG=395062
TEST=manually

Review URL: https://codereview.chromium.org/402863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284654 0039d316-1c4b-4281-b951-d872f2087c98