Add support for building with goma.

README.md

 # PDFium
 
 ## Prerequisites
 
 Get the chromium depot tools via the instructions at
 http://www.chromium.org/developers/how-tos/install-depot-tools (this provides
 the gclient utilty needed below).
 
 Also install Python, Subversion, and Git and make sure they're in your path.
 
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 (http://build.chromium.org/p/client.pdfium/).
 
  * On Windows: `build\gyp_pdfium`
  * For all other platforms: `build/gyp_pdfium`
 
 The second option is to generate platform-specific build files, i.e. Makefiles
 on Linux, sln files on Windows, and xcodeproj files on Mac. To do so, set the
 GYP\_GENERATORS environment variable appropriately (e.g. "make", "msvs", or
 "xcode") before running the above command.
 
+### Using goma (Googlers only)
+
+If you would like to build using goma, pass `use_goma=1` to `gyp_pdfium`. If
+you installed goma in a non-standard location, you will also need to set
+`gomadir`. e.g.
+
+```
+build/gyp_pdfium -D use_goma=1 -D gomadir=path/to/goma
+```
+
 ## Building the code
 
 If you used Ninja, you can build the sample program by: `ninja -C out/Debug
 pdfium_test` You can build the entire product (which includes a few unit
 tests) by: `ninja -C out/Debug`.
 
 If you're not using Ninja, then building is platform-specific.
 
  * On Linux: `make pdfium_test`
  * On Mac: `open build/all.xcodeproj`
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 ```
 git checkout origin/xfa
 git checkout -b merge_branch
 git branch --set-upstream-to=origin/xfa
 git cherry-pick -x <commit hash>
 git commit --amend # add Merge to XFA
 git cl upload
 ```
 
 Then wait for approval, and `git cl land`