Revert of webgl: use immutable texture for the default FBO.

Revert of webgl: use immutable texture for the default FBO. (patchset #2 id:20001 of https://codereview.chromium.org/1449043005/ )

Reason for revert:
following test is broken.
WebglConformance.deqp_functional_gles3_readpixel

Original issue's description:
> webgl: use immutable texture for the default FBO.
>
> Use immutable texture for the default FBO like chromium compositor.
>
> In theory, it speeds up drawing speed, because immutable texture skips texture
> validation on drawing call.
>
> In addition, immutable texture allows GL_INTEL_framebuffer_CMAA more optimization.
>
> BUG=557848
> TEST=
> WebglConformance.conformance_renderbuffers_framebuffer_state_restoration
> WebglConformance.conformance_textures_misc_texture_size_cube_maps
> WebglConformance.conformance_textures_misc_texture_sub_image_cube_maps
> WebglConformance.conformance_textures_webgl_canvas_tex_image_and_sub_image_2d_with_webgl_canvas_rgb_rgb_unsigned_byte
> WebglConformance.conformance_textures_webgl_canvas_tex_image_and_sub_image_2d_with_webgl_canvas_rgb_rgb_unsigned_short_5_6_5
> WebglConformance.conformance_textures_webgl_canvas_tex_image_and_sub_image_2d_with_webgl_canvas_rgba_rgba_unsigned_byte
> WebglConformance.conformance_textures_webgl_canvas_tex_image_and_sub_image_2d_with_webgl_canvas_rgba_rgba_unsigned_short_4_4_4_4
> WebglConformance.conformance_textures_webgl_canvas_tex_image_and_sub_image_2d_with_webgl_canvas_rgba_rgba_unsigned_short_5_5_5_1
>
> Committed: https://crrev.com/fb9d8e245ac84c6c0806e86e7884b4775a84b4b5
> Cr-Commit-Position: refs/heads/master@{#361335}

TBR=kbr@chromium.org,adrian.belgun@intel.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=557848
Committed: https://crrev.com/39ec117eabd4b0b67fe6f004385d36c75503b30c
Cr-Commit-Position: refs/heads/master@{#361368}

[dshwang, 2015-11-24 16:47:47]

Created Revert of webgl: use immutable texture for the default FBO.

[commit-bot: I haz the power, 2015-11-24 16:48:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1474763002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1474763002/1

[commit-bot: I haz the power, 2015-11-24 16:49:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-24 16:50:51]

Patchset 1 (id:??) landed as
https://crrev.com/39ec117eabd4b0b67fe6f004385d36c75503b30c
Cr-Commit-Position: refs/heads/master@{#361368}