Revert "[Backport Patch] Use glDiscardFramebuffer instead of glClear"

There was a follow-up to this patch upstream in crrev.com/656053002
which reverted the performance gains originally obtained.

XWALK-2719 and the original pull request #25 contains more information
about what happened, and crrev.com/667733002 is fixing it in a different
way.

For now, let's wait for everything to settle upstream and go back to the
original M39 behavior.

This reverts commit 90a30f1f6273a6b6bf25263c18ba453c99b6f570.

BUG=XWALK-2719
Merge pull request #26 from rakuco/revert-webgl-opt

Revert "[Backport Patch] Use glDiscardFramebuffer instead of glClear"
Chromium CPU profiler: source line support

This is the second and final part of adding source line support in Chromium CPU profiler that needs to be done to move XDK CPU profiler on the same way as CDT uses.

In brief, we need to do two things:
1) Map the samples to source lines during profiling session and add new V8 public API to get the hit source lines. These changes are already merged into v8-crosswalk.
2) The Chrome profilers works as follows: Chrome Browser ==(a "start/stop profiling" command)==> WebKit (WebSocket) ==> WebKit(InspectorBackendDispatcher) =="start/stop profiling"==> WebKit(InspectorProfilerAgent) ==V8 Public APIs (StartProfiling/Stopprofiling)==> V8.
So, on StopProfiling command, WebKit(InspectorProfilerAgent) gets the collected results (using V8 public API) and packs them into JSON to send back to Chrome browser.

This patch implements writing info about the hit source lines in JSON data to be sent back to Chromium browser.
(cherry picked from commit f16d9209f530d0ab2a2572f0679b14ea7d2f96dc)

Re-enabling CPU profiling feature broken after migration to new v8.
Merge pull request #27 from eleskine/blinkprf-102

Chromium CPU profiler: source line support