Revert of Replace RawPtr with RefPtr on StylePendingImage

Revert of Replace RawPtr with RefPtr on StylePendingImage (patchset #2 id:20001 of https://codereview.chromium.org/1368613002/ )

Reason for revert:
Causing crashes in Canary.

BUG=536655
Original issue's description:
> Replace RawPtr with RefPtr on StylePendingImage
>
> This change ensures StylePendingImages set on ComputedStyles will keep
> their corresponding CSSValue alive.
> The PendingImagePropertyMap on ElementStyleResources was supposed to do
> this however it only keeps alive one per property while properties can
> have any number of images set on them (background-image).
>
> Prior to this change StylePendingImages held a RawPtr to their CSSValue
> to avoid a ref loop because the CSSValue would hold a RefPtr to the
> StylePendingImage as a cache.
>
> After this change StylePendingImages hold RefPtrs to their CSSValue
> while CSSValues no longer cache StylePendingImages. Having multiple
> StylePendingImages per CSSValue is equivalent to using cached
> StylePendingImages as no state changes on the StylePendingImage
> during the StyleResourceLoader pass.
>
> The old scenario is not currently a problem as all applied CSSValues
> have their refs held elsewhere (style rules or AnimatableImages).
> Future animation work intends to apply temporary CSSValue images and
> must ensure StylePendingImages keep a ref to avoid a use after free.
>
> This patch is a refactor towards memory management correctness and has
> no behavioural changes.
>
> BUG=437696
>
> Committed: https://crrev.com/1dd18b7ea8db7d3344024277e04c8a5365c03cf1
> Cr-Commit-Position: refs/heads/master@{#350808}

TBR=timloh@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=437696
Committed: https://crrev.com/950c0aca627fca67413569326c7eaec09c8ffa5d
Cr-Commit-Position: refs/heads/master@{#351024}

[alancutter (OOO until 2018), 2015-09-28 04:58:49]

Created Revert of Replace RawPtr with RefPtr on StylePendingImage

[commit-bot: I haz the power, 2015-09-28 04:59:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1369213002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1369213002/1

[commit-bot: I haz the power, 2015-09-28 04:59:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-28 05:01:09]

Patchset 1 (id:??) landed as
https://crrev.com/950c0aca627fca67413569326c7eaec09c8ffa5d
Cr-Commit-Position: refs/heads/master@{#351024}