Roll WebGL 1eb6562..4951bdd

https://chromium.googlesource.com/external/khronosgroup/webgl.git/+log/1eb6562..4951bdd

BUG=
TEST=bots
TBR=qiankun.miao@intel.com
NOTRY=true
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2158693002
Cr-Commit-Position: refs/heads/master@{#405946}

DEPS

@@ -178,7 +178,7 @@ deps = {
     Var('chromium_git') + '/external/bidichecker/lib.git' + '@' + '97f2aa645b74c28c57eca56992235c79850fa9e0',
 
   'src/third_party/webgl/src':
-   Var('chromium_git') + '/external/khronosgroup/webgl.git' + '@' + '1eb6562d99390f61d1ec9540f15e244dbcd67161',
+   Var('chromium_git') + '/external/khronosgroup/webgl.git' + '@' + '4951bdd3e7aed161dfd4016240b1071ccaaa55c5',
 
   'src/third_party/webdriver/pylib':
     Var('chromium_git') + '/external/selenium/py.git' + '@' + '5fd78261a75fe08d27ca4835fb6c5ce4b42275bd',