Add GL_EXT_shader_implicit_conversions to ES3.1 wrapper ctx
Add GL_EXT_shader_implicit_conversions to ES3.1 wrapper ctx

Underlying GL core context always supports some fashion of implicit
conversions. Thus supporting GL_EXT_shader_implicit_conversions requires
only advertising the extension.

Change-Id: I39be9eab5bb7ab32a4738afe73b374cc059e81b8

maurossi/deqp,KhronosGroup/VK-GL-CTS,geekboxzone/mmallow_external_deqp,maurossi/deqp,maurossi/deqp,KhronosGroup/VK-GL-CTS,geekboxzone/mmallow_external_deqp,geekboxzone/mmallow_external_deqp,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,geekboxzone/mmallow_external_deqp,googlestadia/VK-GL-CTS,maurossi/deqp,maurossi/deqp,chadversary/deqp,chadversary/deqp,chadversary/deqp,chadversary/deqp,KhronosGroup/VK-GL-CTS,chadversary/deqp,chadversary/deqp,maurossi/deqp,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp