Webkit: Null check in CanvasTexture::uploadImageBuffer

This patch adds preventive null check for imageBuffers in
CanvasTexture::uploadImageBuffer.

This code flow can hit when we are not using Gpu Rendering.

Change-Id: I1c6e68fa83ec87b4d4a2850c959c546e20292e5b
CRs-Fixed: 523517
webkit : canvas bug fix. delay canvas paint till its context creation

some times HTMLCanvasElement::paint gets triggered before its
Context is created in JS. Due this GraphicsLayerAndroid assumes
that painting is done, whereas there is no buffer to paint since
the canvas context is not yet initialized.

This patch, don't allows canvas element paint until its context is
created. And once the context is created, we trigger style compute
to initiate paint on canvas element.

Change-Id: I4a985691f0e43f1dcba0c5de93720a5d9ec4e45f
CRs-fixed: 494847