<webview>: Don't reset allowtransparency and allowscaling between createGuest and attachGuest

<webview>: Don't reset allowtransparency and allowscaling between createGuest and attachGuest

https://codereview.chromium.org/1033373003/ introduced lazily issuing updated GuestView attributes
between create and attach. The AllowTransparency and AllowScaling attributes were defaulting to false
if not available in the attach params and so they were getting reset on attach. This CL fixes the issue
by only updating the attributes if they are passed along on attach.

BUG=471997
Committed: https://crrev.com/3f5bfc1e91367878b8d632f62ae482f4205a15a3
Cr-Commit-Position: refs/heads/master@{#324327}

[Fady Samuel, 2015-04-08 20:12:27]

fsamuel@chromium.org changed reviewers:
+ lazyboy@chromium.org

[lazyboy, 2015-04-08 21:29:51]

lgtm

[Fady Samuel, 2015-04-08 21:45:25]

The CQ bit was checked by fsamuel@chromium.org

[commit-bot: I haz the power, 2015-04-08 21:46:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1065413002/1

[commit-bot: I haz the power, 2015-04-09 01:08:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-09 01:09:17]

Patchset 1 (id:??) landed as
https://crrev.com/3f5bfc1e91367878b8d632f62ae482f4205a15a3
Cr-Commit-Position: refs/heads/master@{#324327}