Fix for the flash Text layout framework problems with win32k lockdown mode on Windows 8+

Fix for the flash Text layout framework problems with win32k lockdown mode on Windows 8+

Flash while rendering text calls into pepper to create a canvas and then sends IPCs like PpapiHostMsg_Flash_DrawGlyphs
to draw into the canvas. In Win32K lockdown mode we create a memory canvas instance (ImageDataSimpleBackend). This class
returns NULL in its GetPlatformCanvas implementation which causes the OnDrawGlyphs call to bail.

Fix is to call the GetCanvas function instead of GetPlatformCanvas. Both functions do the same thing.

BUG=468418
R=cpu

Committed: https://crrev.com/df69a28d5b46c2d9a25a14b9fc427a26ecf9f55f
Cr-Commit-Position: refs/heads/master@{#321281}

[cpu_(ooo_6.6-7.5), 2015-03-19 03:01:22]

lgtm

[cpu_(ooo_6.6-7.5), 2015-03-19 03:01:25]

The CQ bit was checked by cpu@chromium.org

[commit-bot: I haz the power, 2015-03-19 03:04:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1007993006/40001

[commit-bot: I haz the power, 2015-03-19 03:11:37]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-03-19 03:12:26]

Patchset 3 (id:??) landed as
https://crrev.com/df69a28d5b46c2d9a25a14b9fc427a26ecf9f55f
Cr-Commit-Position: refs/heads/master@{#321281}