Use GL_LINEAR for backing texture of offscreen framebuffer.

The texture filtering mode of backing texture of offscreen
framebuffer is used for rendering Pepper Graphics3D contents.
If GL_NEAREST is used, the contents looks bad when it is scaled
(e.g. CSS transform, changing display resolution on ChromeOS).
This CL changes the texture filtering mode of backing textures
to GL_LINEAR to prevent the contents from looking bad in such cases.

TEST=Set display resolution to 1024x680 on Pixel
TEST=Apply CSS transform to NaCl emebed element
TEST=Apply CSS transform to Flash content
BUG=384687

Review URL: https://codereview.chromium.org/472443002

git-svn-id: http://src.chromium.org/svn/trunk/src/gpu@290116 4ff67af0-8c30-449e-8e8b-ad334ec8d88c