Code: void WebGLRenderingContextBase::DrawingBufferClientRestorePixelPackAlignment() {
   if (!ContextGL())
     return;
   ContextGL()->PixelStorei(GL_PACK_ALIGNMENT, pack_alignment_);
}

Vulnerability Type: Exec Code Overflow 
CWE ID: CWE-119
Summary: Heap buffer overflow in WebGL in Google Chrome prior to 61.0.3163.79 for Windows allowed a remote attacker to execute arbitrary code inside a sandbox via a crafted HTML page.
Commit Message: Reset ES3 pixel pack parameters and PIXEL_PACK_BUFFER binding in DrawingBuffer before ReadPixels() and recover them later.

BUG=740603
TEST=new conformance test
R=kbr@chromium.org,piman@chromium.org

Change-Id: I3ea54c6cc34f34e249f7c8b9f792d93c5e1958f4
Reviewed-on: https://chromium-review.googlesource.com/570840
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486518}